VOLTAGE SAGS MITIGATION TECHNIQUES ANALYSIS

NORSHAFINASH BINTI SAUDIN

A project report submitted in partial fulfillment of the

requirements for the award of the degree of

Master of Engineering (Electrical ndash Power)

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

JUNE 2007

iii

To my beloved husband

iv

ACKNOWLEDGEMENT

I would like to express my gratitude to Allah SWT for giving me the

opportunity to complete this Masterrsquos Project I am deeply indebted to individuals who

directly or indirectly are responsible for this project

I am most grateful to the most kindheartedness supervisor Dr Ahmad Safawi bin

Mokhtar for his guidance in this project and to panel of seminar presentation PM Dr

Mohd Wazir bin Mustafa and PM Md Shah Majid with their superior guidance

information and ideas for this project become abundance

My admiration falls upon En Saudin bin Mat my father and especially to my

mother Pn Siah binti Taharin for them to bear with me my absence in the family Your

encouragement pray and support are very much appreciated

I would also like to express my sincere thanks to my entire friend for their

support and ideas during the development of the project

And last but not the least to my husband thanks

v

ABSTRACT

For some decades power quality did not cause any problem because it had no

effect on most of the loads connected to the electric distribution system When an

induction motor is subjected to voltage sag the motor still operates but with a lower

output until the sag ends With the increased use of sophisticated electronics high

efficiency variable speed drive and power electronic controller power quality has

become an increasing concern to utilities and customers Voltage sags is the most

common type of power quality disturbance in the distribution system It can be caused

by fault in the electrical network or by the starting of a large induction motor Although

the electric utilities have made a substantial amount of investment to improve the

reliability of the network they cannot control the external factor that causes the fault

such as lightning or accumulation of salt at a transmission tower located near to sea

This project intends to investigate mitigation technique that is suitable for different type

of voltage sags source with different type of loads The simulation will be using

PSCADEMTDC software The mitigation techniques that will be studied are such as

Dynamic Voltage Restorer (DVR) Distribution Static Compensator (DSTATCOM) and

Solid State Transfer Switch (SSTS) All the mitigation techniques will be tested on

different type of faults The analysis will focus on the effectiveness of these techniques

in mitigating the voltage sags The study will also investigate the effects of using the

techniques to phase shift At the end of the project it is expected that a few suggestions

can be made on the suitability of the techniques

vi

ABSTRAK

Beberapa dekad yang lalu kualiti kuasa tidak menjadi permasalahan kerana ia

tidak memberi kesan yang sangat nyata kepada beban yang bersambung dengan sistem

pengagihan Apabila motor aruhan mengalami voltan lendut motor tersebut masih

berfungsi tetapi dengan keluaran yang lebih rendah sehingga kejatuhan voltan tamat

Walau bagaimanapun dengan peningkatan penggunaan peralatan elektronik yang maju

pemacu pelbagai halaju berkecekapan tinggi dan pengawal elektronik kuasa kualiti

kuasa mula menjadi perhatian kepada utiliti dan pelanggan Di mana voltan lendut

adalah gangguan kualiti kuasa yang seringkali terjadi terhadap sistem pengagihan yang

disebabkan oleh kerosakan pada rangkaian elektrik dan pemulaan yang besar untuk

motor aruhan Walaupun utiliti telah membuat pelaburan untuk memperbaiki

keboleharapan rangkaian faktor luaran yang menyebabkan kerosakan masih tidak dapat

dikawal contohnya kilat dan pengumpulan garam pada menara penghantaraan yang

terletak berhampiran dengan laut Oleh itu projek ini bertujuan mengkaji kesesuaian

teknik mitigasi untuk pelbagai punca voltan lendut pada beban yang berbeza di mana

perisian PSCADEMTDC digunakan sebagai bantuan untuk simulasi Teknik - teknik

mitigasi yang dikaji adalah seperti Dynamic Voltage Restorer (DVR) Distribution Static

Compensator (DSTATCOM) dan Solid State Transfer Switch (SSTS) Teknik - teknik ini

akan diuji dengan pelbagai kerosakan yang menyebabkan voltan lendut Tumpuan akan

diberikan kepada keberkesanan teknik-teknik tersebut untuk mengatasi voltan lendut dan

kesannya terhadap anjakan fasa Di akhir projek ini beberapa cadangan akan diutarakan

berkenaan kesesuaian teknik - teknik tersebut digunakan untuk mengatasai voltan lendut

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF ABBREVIATIONS xv

LIST OF APPENDICES xvi

I INTRODUCTION 1

11 Introduction 1

12 Problem Statement 3

13 Project Objectives 6

14 Project Scope 6

viii

II VOLTAGE SAGS 7

21 Introduction 7

22 Definition of Voltage Sags 8

23 Standards Associated with Voltage Sags 9

231 IEEE Standard 10

232 Industry Standard 12

2321 SEMI 12

2322 CBEMA (ITI) Curve 14

24 General Causes and Effects of Voltage Sags 15

241 Voltage Sags due to Faults 15

242 Voltage Sags due to Motor Starting 17

243 Voltage Sags due to Transformer Energizing 18

III PSCADEMTDC SOFTWARE 19

31 Introduction 19

32 Characteristics of Software 20

33 Example of Circuit 22

34 Conclusion 25

ix

IV VOLTAGE SAG MITIGATION TECHNIQUES 26

41 Introduction 26

42 Dynamic Voltage Restorer (DVR) 28

421 Principles of DVR Operation 28

43 Distribution Static Compensator (DSTATCOM) 30

421 Basic Configuration and Function of

DSTATCOM 31

44 Solid State Transfer Switch (SSTS) 34

441 Basic Configuration and Function of SSTS 35

V MITIGATION TECNIQUES REALIZATION 39

51 Sinusoidal PWM-Based Control Scheme 39

52 Test System 42

53 Dynamic Voltage Restorer 43

54 Distribution Static Compensator 45

55 Solid State Transfer Switch 47

x

VI SIMULATIONS AND RESULTS 49

61 Test case 49

62 Single line to ground fault 50

621 Phase A to ground 50

622 Phase B to ground 56

623 Phase C to ground 59

63 Double lines to ground fault 62

631 Phase A and B to ground 62

632 Phase A and C to ground 67

633 Phase B and C to ground 70

64 Conclusion 73

VII CONCLUSION 74

71 Conclusion 74

72 Suggestion 77

REFERENCES 78

Appendices A-C 81-85

xi

LIST OF TABLES

TABLE NO TITLE PAGE

11 Cause of TNB network disruption 4

61 (a) Test results for line A to the ground fault (b) Recovery result 5

62 (a) Test results for line B to the ground fault (b) Recovery result 8

63 (a) Test results for line C to the ground fault (b) Recovery result 1

64 (a) Test results for line AB to the ground fault (b) Recovery result 6

65 (a) Test results for line AC to the ground fault (b) Recovery result 9

66 (a) Test results for line BC to the ground fault (b) Recovery result 2

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

iii

To my beloved husband

iv

ACKNOWLEDGEMENT

I would like to express my gratitude to Allah SWT for giving me the

opportunity to complete this Masterrsquos Project I am deeply indebted to individuals who

directly or indirectly are responsible for this project

I am most grateful to the most kindheartedness supervisor Dr Ahmad Safawi bin

Mokhtar for his guidance in this project and to panel of seminar presentation PM Dr

Mohd Wazir bin Mustafa and PM Md Shah Majid with their superior guidance

information and ideas for this project become abundance

My admiration falls upon En Saudin bin Mat my father and especially to my

mother Pn Siah binti Taharin for them to bear with me my absence in the family Your

encouragement pray and support are very much appreciated

I would also like to express my sincere thanks to my entire friend for their

support and ideas during the development of the project

And last but not the least to my husband thanks

v

ABSTRACT

For some decades power quality did not cause any problem because it had no

effect on most of the loads connected to the electric distribution system When an

induction motor is subjected to voltage sag the motor still operates but with a lower

output until the sag ends With the increased use of sophisticated electronics high

efficiency variable speed drive and power electronic controller power quality has

become an increasing concern to utilities and customers Voltage sags is the most

common type of power quality disturbance in the distribution system It can be caused

by fault in the electrical network or by the starting of a large induction motor Although

the electric utilities have made a substantial amount of investment to improve the

reliability of the network they cannot control the external factor that causes the fault

such as lightning or accumulation of salt at a transmission tower located near to sea

This project intends to investigate mitigation technique that is suitable for different type

of voltage sags source with different type of loads The simulation will be using

PSCADEMTDC software The mitigation techniques that will be studied are such as

Dynamic Voltage Restorer (DVR) Distribution Static Compensator (DSTATCOM) and

Solid State Transfer Switch (SSTS) All the mitigation techniques will be tested on

different type of faults The analysis will focus on the effectiveness of these techniques

in mitigating the voltage sags The study will also investigate the effects of using the

techniques to phase shift At the end of the project it is expected that a few suggestions

can be made on the suitability of the techniques

vi

ABSTRAK

Beberapa dekad yang lalu kualiti kuasa tidak menjadi permasalahan kerana ia

tidak memberi kesan yang sangat nyata kepada beban yang bersambung dengan sistem

pengagihan Apabila motor aruhan mengalami voltan lendut motor tersebut masih

berfungsi tetapi dengan keluaran yang lebih rendah sehingga kejatuhan voltan tamat

Walau bagaimanapun dengan peningkatan penggunaan peralatan elektronik yang maju

pemacu pelbagai halaju berkecekapan tinggi dan pengawal elektronik kuasa kualiti

kuasa mula menjadi perhatian kepada utiliti dan pelanggan Di mana voltan lendut

adalah gangguan kualiti kuasa yang seringkali terjadi terhadap sistem pengagihan yang

disebabkan oleh kerosakan pada rangkaian elektrik dan pemulaan yang besar untuk

motor aruhan Walaupun utiliti telah membuat pelaburan untuk memperbaiki

keboleharapan rangkaian faktor luaran yang menyebabkan kerosakan masih tidak dapat

dikawal contohnya kilat dan pengumpulan garam pada menara penghantaraan yang

terletak berhampiran dengan laut Oleh itu projek ini bertujuan mengkaji kesesuaian

teknik mitigasi untuk pelbagai punca voltan lendut pada beban yang berbeza di mana

perisian PSCADEMTDC digunakan sebagai bantuan untuk simulasi Teknik - teknik

mitigasi yang dikaji adalah seperti Dynamic Voltage Restorer (DVR) Distribution Static

Compensator (DSTATCOM) dan Solid State Transfer Switch (SSTS) Teknik - teknik ini

akan diuji dengan pelbagai kerosakan yang menyebabkan voltan lendut Tumpuan akan

diberikan kepada keberkesanan teknik-teknik tersebut untuk mengatasi voltan lendut dan

kesannya terhadap anjakan fasa Di akhir projek ini beberapa cadangan akan diutarakan

berkenaan kesesuaian teknik - teknik tersebut digunakan untuk mengatasai voltan lendut

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF ABBREVIATIONS xv

LIST OF APPENDICES xvi

I INTRODUCTION 1

11 Introduction 1

12 Problem Statement 3

13 Project Objectives 6

14 Project Scope 6

viii

II VOLTAGE SAGS 7

21 Introduction 7

22 Definition of Voltage Sags 8

23 Standards Associated with Voltage Sags 9

231 IEEE Standard 10

232 Industry Standard 12

2321 SEMI 12

2322 CBEMA (ITI) Curve 14

24 General Causes and Effects of Voltage Sags 15

241 Voltage Sags due to Faults 15

242 Voltage Sags due to Motor Starting 17

243 Voltage Sags due to Transformer Energizing 18

III PSCADEMTDC SOFTWARE 19

31 Introduction 19

32 Characteristics of Software 20

33 Example of Circuit 22

34 Conclusion 25

ix

IV VOLTAGE SAG MITIGATION TECHNIQUES 26

41 Introduction 26

42 Dynamic Voltage Restorer (DVR) 28

421 Principles of DVR Operation 28

43 Distribution Static Compensator (DSTATCOM) 30

421 Basic Configuration and Function of

DSTATCOM 31

44 Solid State Transfer Switch (SSTS) 34

441 Basic Configuration and Function of SSTS 35

V MITIGATION TECNIQUES REALIZATION 39

51 Sinusoidal PWM-Based Control Scheme 39

52 Test System 42

53 Dynamic Voltage Restorer 43

54 Distribution Static Compensator 45

55 Solid State Transfer Switch 47

x

VI SIMULATIONS AND RESULTS 49

61 Test case 49

62 Single line to ground fault 50

621 Phase A to ground 50

622 Phase B to ground 56

623 Phase C to ground 59

63 Double lines to ground fault 62

631 Phase A and B to ground 62

632 Phase A and C to ground 67

633 Phase B and C to ground 70

64 Conclusion 73

VII CONCLUSION 74

71 Conclusion 74

72 Suggestion 77

REFERENCES 78

Appendices A-C 81-85

xi

LIST OF TABLES

TABLE NO TITLE PAGE

11 Cause of TNB network disruption 4

61 (a) Test results for line A to the ground fault (b) Recovery result 5

62 (a) Test results for line B to the ground fault (b) Recovery result 8

63 (a) Test results for line C to the ground fault (b) Recovery result 1

64 (a) Test results for line AB to the ground fault (b) Recovery result 6

65 (a) Test results for line AC to the ground fault (b) Recovery result 9

66 (a) Test results for line BC to the ground fault (b) Recovery result 2

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

iv

ACKNOWLEDGEMENT

I would like to express my gratitude to Allah SWT for giving me the

opportunity to complete this Masterrsquos Project I am deeply indebted to individuals who

directly or indirectly are responsible for this project

I am most grateful to the most kindheartedness supervisor Dr Ahmad Safawi bin

Mokhtar for his guidance in this project and to panel of seminar presentation PM Dr

Mohd Wazir bin Mustafa and PM Md Shah Majid with their superior guidance

information and ideas for this project become abundance

My admiration falls upon En Saudin bin Mat my father and especially to my

mother Pn Siah binti Taharin for them to bear with me my absence in the family Your

encouragement pray and support are very much appreciated

I would also like to express my sincere thanks to my entire friend for their

support and ideas during the development of the project

And last but not the least to my husband thanks

v

ABSTRACT

For some decades power quality did not cause any problem because it had no

effect on most of the loads connected to the electric distribution system When an

induction motor is subjected to voltage sag the motor still operates but with a lower

output until the sag ends With the increased use of sophisticated electronics high

efficiency variable speed drive and power electronic controller power quality has

become an increasing concern to utilities and customers Voltage sags is the most

common type of power quality disturbance in the distribution system It can be caused

by fault in the electrical network or by the starting of a large induction motor Although

the electric utilities have made a substantial amount of investment to improve the

reliability of the network they cannot control the external factor that causes the fault

such as lightning or accumulation of salt at a transmission tower located near to sea

This project intends to investigate mitigation technique that is suitable for different type

of voltage sags source with different type of loads The simulation will be using

PSCADEMTDC software The mitigation techniques that will be studied are such as

Dynamic Voltage Restorer (DVR) Distribution Static Compensator (DSTATCOM) and

Solid State Transfer Switch (SSTS) All the mitigation techniques will be tested on

different type of faults The analysis will focus on the effectiveness of these techniques

in mitigating the voltage sags The study will also investigate the effects of using the

techniques to phase shift At the end of the project it is expected that a few suggestions

can be made on the suitability of the techniques

vi

ABSTRAK

Beberapa dekad yang lalu kualiti kuasa tidak menjadi permasalahan kerana ia

tidak memberi kesan yang sangat nyata kepada beban yang bersambung dengan sistem

pengagihan Apabila motor aruhan mengalami voltan lendut motor tersebut masih

berfungsi tetapi dengan keluaran yang lebih rendah sehingga kejatuhan voltan tamat

Walau bagaimanapun dengan peningkatan penggunaan peralatan elektronik yang maju

pemacu pelbagai halaju berkecekapan tinggi dan pengawal elektronik kuasa kualiti

kuasa mula menjadi perhatian kepada utiliti dan pelanggan Di mana voltan lendut

adalah gangguan kualiti kuasa yang seringkali terjadi terhadap sistem pengagihan yang

disebabkan oleh kerosakan pada rangkaian elektrik dan pemulaan yang besar untuk

motor aruhan Walaupun utiliti telah membuat pelaburan untuk memperbaiki

keboleharapan rangkaian faktor luaran yang menyebabkan kerosakan masih tidak dapat

dikawal contohnya kilat dan pengumpulan garam pada menara penghantaraan yang

terletak berhampiran dengan laut Oleh itu projek ini bertujuan mengkaji kesesuaian

teknik mitigasi untuk pelbagai punca voltan lendut pada beban yang berbeza di mana

perisian PSCADEMTDC digunakan sebagai bantuan untuk simulasi Teknik - teknik

mitigasi yang dikaji adalah seperti Dynamic Voltage Restorer (DVR) Distribution Static

Compensator (DSTATCOM) dan Solid State Transfer Switch (SSTS) Teknik - teknik ini

akan diuji dengan pelbagai kerosakan yang menyebabkan voltan lendut Tumpuan akan

diberikan kepada keberkesanan teknik-teknik tersebut untuk mengatasi voltan lendut dan

kesannya terhadap anjakan fasa Di akhir projek ini beberapa cadangan akan diutarakan

berkenaan kesesuaian teknik - teknik tersebut digunakan untuk mengatasai voltan lendut

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF ABBREVIATIONS xv

LIST OF APPENDICES xvi

I INTRODUCTION 1

11 Introduction 1

12 Problem Statement 3

13 Project Objectives 6

14 Project Scope 6

viii

II VOLTAGE SAGS 7

21 Introduction 7

22 Definition of Voltage Sags 8

23 Standards Associated with Voltage Sags 9

231 IEEE Standard 10

232 Industry Standard 12

2321 SEMI 12

2322 CBEMA (ITI) Curve 14

24 General Causes and Effects of Voltage Sags 15

241 Voltage Sags due to Faults 15

242 Voltage Sags due to Motor Starting 17

243 Voltage Sags due to Transformer Energizing 18

III PSCADEMTDC SOFTWARE 19

31 Introduction 19

32 Characteristics of Software 20

33 Example of Circuit 22

34 Conclusion 25

ix

IV VOLTAGE SAG MITIGATION TECHNIQUES 26

41 Introduction 26

42 Dynamic Voltage Restorer (DVR) 28

421 Principles of DVR Operation 28

43 Distribution Static Compensator (DSTATCOM) 30

421 Basic Configuration and Function of

DSTATCOM 31

44 Solid State Transfer Switch (SSTS) 34

441 Basic Configuration and Function of SSTS 35

V MITIGATION TECNIQUES REALIZATION 39

51 Sinusoidal PWM-Based Control Scheme 39

52 Test System 42

53 Dynamic Voltage Restorer 43

54 Distribution Static Compensator 45

55 Solid State Transfer Switch 47

x

VI SIMULATIONS AND RESULTS 49

61 Test case 49

62 Single line to ground fault 50

621 Phase A to ground 50

622 Phase B to ground 56

623 Phase C to ground 59

63 Double lines to ground fault 62

631 Phase A and B to ground 62

632 Phase A and C to ground 67

633 Phase B and C to ground 70

64 Conclusion 73

VII CONCLUSION 74

71 Conclusion 74

72 Suggestion 77

REFERENCES 78

Appendices A-C 81-85

xi

LIST OF TABLES

TABLE NO TITLE PAGE

11 Cause of TNB network disruption 4

61 (a) Test results for line A to the ground fault (b) Recovery result 5

62 (a) Test results for line B to the ground fault (b) Recovery result 8

63 (a) Test results for line C to the ground fault (b) Recovery result 1

64 (a) Test results for line AB to the ground fault (b) Recovery result 6

65 (a) Test results for line AC to the ground fault (b) Recovery result 9

66 (a) Test results for line BC to the ground fault (b) Recovery result 2

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

v

ABSTRACT

For some decades power quality did not cause any problem because it had no

effect on most of the loads connected to the electric distribution system When an

induction motor is subjected to voltage sag the motor still operates but with a lower

output until the sag ends With the increased use of sophisticated electronics high

efficiency variable speed drive and power electronic controller power quality has

become an increasing concern to utilities and customers Voltage sags is the most

common type of power quality disturbance in the distribution system It can be caused

by fault in the electrical network or by the starting of a large induction motor Although

the electric utilities have made a substantial amount of investment to improve the

reliability of the network they cannot control the external factor that causes the fault

such as lightning or accumulation of salt at a transmission tower located near to sea

This project intends to investigate mitigation technique that is suitable for different type

of voltage sags source with different type of loads The simulation will be using

PSCADEMTDC software The mitigation techniques that will be studied are such as

Dynamic Voltage Restorer (DVR) Distribution Static Compensator (DSTATCOM) and

Solid State Transfer Switch (SSTS) All the mitigation techniques will be tested on

different type of faults The analysis will focus on the effectiveness of these techniques

in mitigating the voltage sags The study will also investigate the effects of using the

techniques to phase shift At the end of the project it is expected that a few suggestions

can be made on the suitability of the techniques

vi

ABSTRAK

Beberapa dekad yang lalu kualiti kuasa tidak menjadi permasalahan kerana ia

tidak memberi kesan yang sangat nyata kepada beban yang bersambung dengan sistem

pengagihan Apabila motor aruhan mengalami voltan lendut motor tersebut masih

berfungsi tetapi dengan keluaran yang lebih rendah sehingga kejatuhan voltan tamat

Walau bagaimanapun dengan peningkatan penggunaan peralatan elektronik yang maju

pemacu pelbagai halaju berkecekapan tinggi dan pengawal elektronik kuasa kualiti

kuasa mula menjadi perhatian kepada utiliti dan pelanggan Di mana voltan lendut

adalah gangguan kualiti kuasa yang seringkali terjadi terhadap sistem pengagihan yang

disebabkan oleh kerosakan pada rangkaian elektrik dan pemulaan yang besar untuk

motor aruhan Walaupun utiliti telah membuat pelaburan untuk memperbaiki

keboleharapan rangkaian faktor luaran yang menyebabkan kerosakan masih tidak dapat

dikawal contohnya kilat dan pengumpulan garam pada menara penghantaraan yang

terletak berhampiran dengan laut Oleh itu projek ini bertujuan mengkaji kesesuaian

teknik mitigasi untuk pelbagai punca voltan lendut pada beban yang berbeza di mana

perisian PSCADEMTDC digunakan sebagai bantuan untuk simulasi Teknik - teknik

mitigasi yang dikaji adalah seperti Dynamic Voltage Restorer (DVR) Distribution Static

Compensator (DSTATCOM) dan Solid State Transfer Switch (SSTS) Teknik - teknik ini

akan diuji dengan pelbagai kerosakan yang menyebabkan voltan lendut Tumpuan akan

diberikan kepada keberkesanan teknik-teknik tersebut untuk mengatasi voltan lendut dan

kesannya terhadap anjakan fasa Di akhir projek ini beberapa cadangan akan diutarakan

berkenaan kesesuaian teknik - teknik tersebut digunakan untuk mengatasai voltan lendut

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF ABBREVIATIONS xv

LIST OF APPENDICES xvi

I INTRODUCTION 1

11 Introduction 1

12 Problem Statement 3

13 Project Objectives 6

14 Project Scope 6

viii

II VOLTAGE SAGS 7

21 Introduction 7

22 Definition of Voltage Sags 8

23 Standards Associated with Voltage Sags 9

231 IEEE Standard 10

232 Industry Standard 12

2321 SEMI 12

2322 CBEMA (ITI) Curve 14

24 General Causes and Effects of Voltage Sags 15

241 Voltage Sags due to Faults 15

242 Voltage Sags due to Motor Starting 17

243 Voltage Sags due to Transformer Energizing 18

III PSCADEMTDC SOFTWARE 19

31 Introduction 19

32 Characteristics of Software 20

33 Example of Circuit 22

34 Conclusion 25

ix

IV VOLTAGE SAG MITIGATION TECHNIQUES 26

41 Introduction 26

42 Dynamic Voltage Restorer (DVR) 28

421 Principles of DVR Operation 28

43 Distribution Static Compensator (DSTATCOM) 30

421 Basic Configuration and Function of

DSTATCOM 31

44 Solid State Transfer Switch (SSTS) 34

441 Basic Configuration and Function of SSTS 35

V MITIGATION TECNIQUES REALIZATION 39

51 Sinusoidal PWM-Based Control Scheme 39

52 Test System 42

53 Dynamic Voltage Restorer 43

54 Distribution Static Compensator 45

55 Solid State Transfer Switch 47

x

VI SIMULATIONS AND RESULTS 49

61 Test case 49

62 Single line to ground fault 50

621 Phase A to ground 50

622 Phase B to ground 56

623 Phase C to ground 59

63 Double lines to ground fault 62

631 Phase A and B to ground 62

632 Phase A and C to ground 67

633 Phase B and C to ground 70

64 Conclusion 73

VII CONCLUSION 74

71 Conclusion 74

72 Suggestion 77

REFERENCES 78

Appendices A-C 81-85

xi

LIST OF TABLES

TABLE NO TITLE PAGE

11 Cause of TNB network disruption 4

61 (a) Test results for line A to the ground fault (b) Recovery result 5

62 (a) Test results for line B to the ground fault (b) Recovery result 8

63 (a) Test results for line C to the ground fault (b) Recovery result 1

64 (a) Test results for line AB to the ground fault (b) Recovery result 6

65 (a) Test results for line AC to the ground fault (b) Recovery result 9

66 (a) Test results for line BC to the ground fault (b) Recovery result 2

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

vi

ABSTRAK

Beberapa dekad yang lalu kualiti kuasa tidak menjadi permasalahan kerana ia

tidak memberi kesan yang sangat nyata kepada beban yang bersambung dengan sistem

pengagihan Apabila motor aruhan mengalami voltan lendut motor tersebut masih

berfungsi tetapi dengan keluaran yang lebih rendah sehingga kejatuhan voltan tamat

Walau bagaimanapun dengan peningkatan penggunaan peralatan elektronik yang maju

pemacu pelbagai halaju berkecekapan tinggi dan pengawal elektronik kuasa kualiti

kuasa mula menjadi perhatian kepada utiliti dan pelanggan Di mana voltan lendut

adalah gangguan kualiti kuasa yang seringkali terjadi terhadap sistem pengagihan yang

disebabkan oleh kerosakan pada rangkaian elektrik dan pemulaan yang besar untuk

motor aruhan Walaupun utiliti telah membuat pelaburan untuk memperbaiki

keboleharapan rangkaian faktor luaran yang menyebabkan kerosakan masih tidak dapat

dikawal contohnya kilat dan pengumpulan garam pada menara penghantaraan yang

terletak berhampiran dengan laut Oleh itu projek ini bertujuan mengkaji kesesuaian

teknik mitigasi untuk pelbagai punca voltan lendut pada beban yang berbeza di mana

perisian PSCADEMTDC digunakan sebagai bantuan untuk simulasi Teknik - teknik

mitigasi yang dikaji adalah seperti Dynamic Voltage Restorer (DVR) Distribution Static

Compensator (DSTATCOM) dan Solid State Transfer Switch (SSTS) Teknik - teknik ini

akan diuji dengan pelbagai kerosakan yang menyebabkan voltan lendut Tumpuan akan

diberikan kepada keberkesanan teknik-teknik tersebut untuk mengatasi voltan lendut dan

kesannya terhadap anjakan fasa Di akhir projek ini beberapa cadangan akan diutarakan

berkenaan kesesuaian teknik - teknik tersebut digunakan untuk mengatasai voltan lendut

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF ABBREVIATIONS xv

LIST OF APPENDICES xvi

I INTRODUCTION 1

11 Introduction 1

12 Problem Statement 3

13 Project Objectives 6

14 Project Scope 6

viii

II VOLTAGE SAGS 7

21 Introduction 7

22 Definition of Voltage Sags 8

23 Standards Associated with Voltage Sags 9

231 IEEE Standard 10

232 Industry Standard 12

2321 SEMI 12

2322 CBEMA (ITI) Curve 14

24 General Causes and Effects of Voltage Sags 15

241 Voltage Sags due to Faults 15

242 Voltage Sags due to Motor Starting 17

243 Voltage Sags due to Transformer Energizing 18

III PSCADEMTDC SOFTWARE 19

31 Introduction 19

32 Characteristics of Software 20

33 Example of Circuit 22

34 Conclusion 25

ix

IV VOLTAGE SAG MITIGATION TECHNIQUES 26

41 Introduction 26

42 Dynamic Voltage Restorer (DVR) 28

421 Principles of DVR Operation 28

43 Distribution Static Compensator (DSTATCOM) 30

421 Basic Configuration and Function of

DSTATCOM 31

44 Solid State Transfer Switch (SSTS) 34

441 Basic Configuration and Function of SSTS 35

V MITIGATION TECNIQUES REALIZATION 39

51 Sinusoidal PWM-Based Control Scheme 39

52 Test System 42

53 Dynamic Voltage Restorer 43

54 Distribution Static Compensator 45

55 Solid State Transfer Switch 47

x

VI SIMULATIONS AND RESULTS 49

61 Test case 49

62 Single line to ground fault 50

621 Phase A to ground 50

622 Phase B to ground 56

623 Phase C to ground 59

63 Double lines to ground fault 62

631 Phase A and B to ground 62

632 Phase A and C to ground 67

633 Phase B and C to ground 70

64 Conclusion 73

VII CONCLUSION 74

71 Conclusion 74

72 Suggestion 77

REFERENCES 78

Appendices A-C 81-85

xi

LIST OF TABLES

TABLE NO TITLE PAGE

11 Cause of TNB network disruption 4

61 (a) Test results for line A to the ground fault (b) Recovery result 5

62 (a) Test results for line B to the ground fault (b) Recovery result 8

63 (a) Test results for line C to the ground fault (b) Recovery result 1

64 (a) Test results for line AB to the ground fault (b) Recovery result 6

65 (a) Test results for line AC to the ground fault (b) Recovery result 9

66 (a) Test results for line BC to the ground fault (b) Recovery result 2

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF ABBREVIATIONS xv

LIST OF APPENDICES xvi

I INTRODUCTION 1

11 Introduction 1

12 Problem Statement 3

13 Project Objectives 6

14 Project Scope 6

viii

II VOLTAGE SAGS 7

21 Introduction 7

22 Definition of Voltage Sags 8

23 Standards Associated with Voltage Sags 9

231 IEEE Standard 10

232 Industry Standard 12

2321 SEMI 12

2322 CBEMA (ITI) Curve 14

24 General Causes and Effects of Voltage Sags 15

241 Voltage Sags due to Faults 15

242 Voltage Sags due to Motor Starting 17

243 Voltage Sags due to Transformer Energizing 18

III PSCADEMTDC SOFTWARE 19

31 Introduction 19

32 Characteristics of Software 20

33 Example of Circuit 22

34 Conclusion 25

ix

IV VOLTAGE SAG MITIGATION TECHNIQUES 26

41 Introduction 26

42 Dynamic Voltage Restorer (DVR) 28

421 Principles of DVR Operation 28

43 Distribution Static Compensator (DSTATCOM) 30

421 Basic Configuration and Function of

DSTATCOM 31

44 Solid State Transfer Switch (SSTS) 34

441 Basic Configuration and Function of SSTS 35

V MITIGATION TECNIQUES REALIZATION 39

51 Sinusoidal PWM-Based Control Scheme 39

52 Test System 42

53 Dynamic Voltage Restorer 43

54 Distribution Static Compensator 45

55 Solid State Transfer Switch 47

x

VI SIMULATIONS AND RESULTS 49

61 Test case 49

62 Single line to ground fault 50

621 Phase A to ground 50

622 Phase B to ground 56

623 Phase C to ground 59

63 Double lines to ground fault 62

631 Phase A and B to ground 62

632 Phase A and C to ground 67

633 Phase B and C to ground 70

64 Conclusion 73

VII CONCLUSION 74

71 Conclusion 74

72 Suggestion 77

REFERENCES 78

Appendices A-C 81-85

xi

LIST OF TABLES

TABLE NO TITLE PAGE

11 Cause of TNB network disruption 4

61 (a) Test results for line A to the ground fault (b) Recovery result 5

62 (a) Test results for line B to the ground fault (b) Recovery result 8

63 (a) Test results for line C to the ground fault (b) Recovery result 1

64 (a) Test results for line AB to the ground fault (b) Recovery result 6

65 (a) Test results for line AC to the ground fault (b) Recovery result 9

66 (a) Test results for line BC to the ground fault (b) Recovery result 2

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

viii

II VOLTAGE SAGS 7

21 Introduction 7

22 Definition of Voltage Sags 8

23 Standards Associated with Voltage Sags 9

231 IEEE Standard 10

232 Industry Standard 12

2321 SEMI 12

2322 CBEMA (ITI) Curve 14

24 General Causes and Effects of Voltage Sags 15

241 Voltage Sags due to Faults 15

242 Voltage Sags due to Motor Starting 17

243 Voltage Sags due to Transformer Energizing 18

III PSCADEMTDC SOFTWARE 19

31 Introduction 19

32 Characteristics of Software 20

33 Example of Circuit 22

34 Conclusion 25

ix

IV VOLTAGE SAG MITIGATION TECHNIQUES 26

41 Introduction 26

42 Dynamic Voltage Restorer (DVR) 28

421 Principles of DVR Operation 28

43 Distribution Static Compensator (DSTATCOM) 30

421 Basic Configuration and Function of

DSTATCOM 31

44 Solid State Transfer Switch (SSTS) 34

441 Basic Configuration and Function of SSTS 35

V MITIGATION TECNIQUES REALIZATION 39

51 Sinusoidal PWM-Based Control Scheme 39

52 Test System 42

53 Dynamic Voltage Restorer 43

54 Distribution Static Compensator 45

55 Solid State Transfer Switch 47

x

VI SIMULATIONS AND RESULTS 49

61 Test case 49

62 Single line to ground fault 50

621 Phase A to ground 50

622 Phase B to ground 56

623 Phase C to ground 59

63 Double lines to ground fault 62

631 Phase A and B to ground 62

632 Phase A and C to ground 67

633 Phase B and C to ground 70

64 Conclusion 73

VII CONCLUSION 74

71 Conclusion 74

72 Suggestion 77

REFERENCES 78

Appendices A-C 81-85

xi

LIST OF TABLES

TABLE NO TITLE PAGE

11 Cause of TNB network disruption 4

61 (a) Test results for line A to the ground fault (b) Recovery result 5

62 (a) Test results for line B to the ground fault (b) Recovery result 8

63 (a) Test results for line C to the ground fault (b) Recovery result 1

64 (a) Test results for line AB to the ground fault (b) Recovery result 6

65 (a) Test results for line AC to the ground fault (b) Recovery result 9

66 (a) Test results for line BC to the ground fault (b) Recovery result 2

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

ix

IV VOLTAGE SAG MITIGATION TECHNIQUES 26

41 Introduction 26

42 Dynamic Voltage Restorer (DVR) 28

421 Principles of DVR Operation 28

43 Distribution Static Compensator (DSTATCOM) 30

421 Basic Configuration and Function of

DSTATCOM 31

44 Solid State Transfer Switch (SSTS) 34

441 Basic Configuration and Function of SSTS 35

V MITIGATION TECNIQUES REALIZATION 39

51 Sinusoidal PWM-Based Control Scheme 39

52 Test System 42

53 Dynamic Voltage Restorer 43

54 Distribution Static Compensator 45

55 Solid State Transfer Switch 47

x

VI SIMULATIONS AND RESULTS 49

61 Test case 49

62 Single line to ground fault 50

621 Phase A to ground 50

622 Phase B to ground 56

623 Phase C to ground 59

63 Double lines to ground fault 62

631 Phase A and B to ground 62

632 Phase A and C to ground 67

633 Phase B and C to ground 70

64 Conclusion 73

VII CONCLUSION 74

71 Conclusion 74

72 Suggestion 77

REFERENCES 78

Appendices A-C 81-85

xi

LIST OF TABLES

TABLE NO TITLE PAGE

11 Cause of TNB network disruption 4

61 (a) Test results for line A to the ground fault (b) Recovery result 5

62 (a) Test results for line B to the ground fault (b) Recovery result 8

63 (a) Test results for line C to the ground fault (b) Recovery result 1

64 (a) Test results for line AB to the ground fault (b) Recovery result 6

65 (a) Test results for line AC to the ground fault (b) Recovery result 9

66 (a) Test results for line BC to the ground fault (b) Recovery result 2

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

x

VI SIMULATIONS AND RESULTS 49

61 Test case 49

62 Single line to ground fault 50

621 Phase A to ground 50

622 Phase B to ground 56

623 Phase C to ground 59

63 Double lines to ground fault 62

631 Phase A and B to ground 62

632 Phase A and C to ground 67

633 Phase B and C to ground 70

64 Conclusion 73

VII CONCLUSION 74

71 Conclusion 74

72 Suggestion 77

REFERENCES 78

Appendices A-C 81-85

xi

LIST OF TABLES

TABLE NO TITLE PAGE

11 Cause of TNB network disruption 4

61 (a) Test results for line A to the ground fault (b) Recovery result 5

62 (a) Test results for line B to the ground fault (b) Recovery result 8

63 (a) Test results for line C to the ground fault (b) Recovery result 1

64 (a) Test results for line AB to the ground fault (b) Recovery result 6

65 (a) Test results for line AC to the ground fault (b) Recovery result 9

66 (a) Test results for line BC to the ground fault (b) Recovery result 2

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

xi

LIST OF TABLES

TABLE NO TITLE PAGE

11 Cause of TNB network disruption 4

61 (a) Test results for line A to the ground fault (b) Recovery result 5

62 (a) Test results for line B to the ground fault (b) Recovery result 8

63 (a) Test results for line C to the ground fault (b) Recovery result 1

64 (a) Test results for line AB to the ground fault (b) Recovery result 6

65 (a) Test results for line AC to the ground fault (b) Recovery result 9

66 (a) Test results for line BC to the ground fault (b) Recovery result 2

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

xii

LIST OF FIGURES

FIGURE NO TITLE PAGE

11 Demarcation of the various power quality issues defined

by IEEE Std 1159-1995 2

21 Depiction of voltage sag 9

22 Immunity curve for semiconductor manufacturing

equipment according to SEMI F47 13

23 Revised CBEMA curve ITIC curve 1996 14

24 Voltage sag due to a cleared line-ground fault 16

25 Voltage sag due to motor starting 17

26 Voltage sag due to transformer energizing 18

31 DVR with main components in PSCAD 23

32 The Wye-Connected DVR in PSCAD 24

41 Different protection options for improving performance during

power quality variation 27

42 Principle of DVR with a response time of less than one

millisecond 29

43 Schematic diagram of the DSTATCOM as a custom

power controller 30

44 Building blocks of DSTATCOM 32

45 Operation modes of a DSTATCOM 33

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

xiii

46 Schematic representations of the SSTS as a custom power device 34

47 Solid State Transfer Switch systems 35

48 Thyristors of the SSTS conducting in the positive and

negative half cycle of the preferred source 37

49 Thyristors on the alternate supply are turned ON on sensing

a disturbance on the preferred source 38

51 Control scheme for the test system implemented in

PSCADEMTDC to carry out the DSTATCOM and DVR

simulations 40

52 The test system implemented in PSCADEMTDC 42

53 One line diagram of the DVR test system 43

54 Schematic diagram of the DVR 44

55 Schematic diagram of the test system with DVR connected

to the system 44

56 One line diagram of the DSTATCOM test system 45

57 Schematic diagram of the test system with DSTATCOM

connected to the system 46

58 One line diagram of the SSTS test system 47

59 SSTS switches implemented in PSCADEMTDC 48

510 Schematic diagram of the test system with SSTS connected

to the system 48

61 (a) Phase shift for line A to the ground fault

(b) Rms voltage drop 50

62 (a) Corrected phase with DVR

(b) Compensated voltage sag with DVR 51

63 (a) Corrected phase using DSTATCOM

(b) Compensated voltage sag using DSTATCOM 53

64 (a) Corrected phase using SSTS

(b) Compensated voltage sag using SSTS 54

65 Phase shift of line B to the ground fault 56

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

xiv

66 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B to

the ground fault 57

67 Phase shift of line B to the ground fault 59

68 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line C to

the ground fault 60

69 (a) Phase shift for line A and B to the ground fault

(b) Rms voltage drop 63

610 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and B

to the ground fault 64

611 (a) Compensated voltage sag using DVR

(b) Compensated voltage sag using DSTATCOM

Line A and B to the ground fault 65

612 Phase shift for line A and C to the ground fault 67

613 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line A and C

to the ground fault 68

614 Phase shift for line B and C to the ground fault 70

615 (a) Phase correction using DVR

(b) Phase correction using DSTATCOM line B and C

to the ground fault 71

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

xv

LIST OF ABBREVIATIONS

CBEMA - Computer Business Equipment Manufacturers Association

DSTATCOM - Distribution Static Compensator

DVR - Dynamic Voltage Restorer

EMTDC - Electromagnetic Transient Program with DC Analysis

ERM - Electronic Restart Modules

Hz - Hertz

IEC - International Electrotechnical Commission

IEEE - Institute of Electrical and Electronics Engineers

ITIC - Information Technology Industry Council

kV - kilovolt

MVA - megavolt ampere

MVAR - mega volt amps reactive

MW - megawatt

pu - per unit

PCC - point of common coupling

PSCAD - Power System Aided Design

PWM - Pulse Width Modulation

RMS - root mean square

SEMI - Semiconductor Equipment and Materials International

SSTS - Solid State Transfer Switch

TNB - Tenaga Nasional Berhad

TRV - transient recovery voltage

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

xvi

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data generated by PSCADEMTDC for DSTATCOM 81

B Data generated by PSCADEMTDC for DVR 83

C Data generated by PSCADEMTDC for SSTS 85

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

CHAPTER I

INTRODUCTION

11 Introduction

Both electric utilities and end users of electrical power are becoming increasingly

concerned about the quality of electric power The term power quality has become one

of the most prolific buzzword in the power industry since the late 1980s [1] The issue in

electricity power sector delivery is not confined to only energy efficiency and

environment but more importantly on quality and continuity of supply or power quality

and supply quality Electrical Power quality is the degree of any deviation from the

nominal values of the voltage magnitude and frequency Power quality may also be

defined as the degree to which both the utilization and delivery of electric power affects

the performance of electrical equipment [2] From a customer perspective a power

quality problem is defined as any power problem manifested in voltage current or

frequency deviations that result in power failure or disoperation of customer of

equipment [3]

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

2

Power quality problems concerning frequency deviation are the presence of

harmonics and other departures from the intended frequency of the alternating supply

voltage On the other hand power quality problems concerning voltage magnitude

deviations can be in the form of voltage fluctuations especially those causing flicker

Other voltage problems are the voltage sags short interruptions and transient over

voltages Transient over voltage has some of the characteristics of high-frequency

phenomena In a three-phase system unbalanced voltages also is a power quality

problem [2] Among them two power quality problems have been identified to be of

major concern to the customers are voltage sags and harmonics but this project will be

focusing on voltage sags

Figures 11 describe the demarcation of the various power quality issues defined

by IEEE Std 1159-1995 [4]

Figure 11 Demarcation of the various power quality issues defined by IEEE

Std 1159-1995[4]

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

3

Three factors that are driving interest and serious concerns in power quality are

[1]

i Increased load sensitivity and production automation The focus on

power quality is therefore more of voltage quality as the momentary drop

in voltage disrupts automated manufacturing processes

ii Automation and efficiency relies on digital components which requires dc

supply As public utilities supply ac power dc power supplies powered

by ac are needed by the dc loads

iii As more dc power supply are needed the converters that convert ac to dc

cause harmonics to be injected into the system and hence reduce wave

form quality

12 Problem Statement

With the increased use of sophisticated electronics high efficiency variable

speed drive and power electronic controller power quality has become an increasing

concern to utilities and customers Voltage sags is the most common type of power

quality disturbance in the distribution system It can be caused by fault in the electrical

network or by the starting of a large induction motor Although the electric utilities have

made a substantial amount of investment to improve the reliability of the network they

cannot control the external factor that causes the fault such as lightning or accumulation

of salt at a transmission tower located near to sea

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

4

Meanwhile during short circuits bus voltages throughout the supply network are

depressed severities of which are dependent of the distance from each bus to point

where the short circuit occurs After clearance of the fault by the protective system the

voltages return to their new steady state values Part of the circuit that is cleared will

suffer supply disruption or blackout Thus in general a short circuit will cause voltage

sags throughout the system but cause blackout to a small portion of the network [1]

A comprehensive study on the cost of losses due to power quality problem has

not been carried out yet However it has been reported that a petrochemical based

industries customer in the Tenaga Nasional Berhad Malaysia system can lose up to

RM164000 (US$43000) per incident related to power quality problem due to voltage

sag Another semiconductor-based industry in the Klang Valley has estimated the loss of

RM5million for the year 2000 Other types of industries such the cement and garment

industries in Malaysia have also reported huge losses due power quality problems One

cement plant has reported an average loss of RM300 000 per incident [2]

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

5

Table 11 Cause of TNB network disruption [2]

In general voltage sags can causes

i Motor load to stallstop

ii Digital devices to reset causing loss of data

iii Equipment damage andor failure

iv Materials Spoilage

v Lost production due to downtime

vi Additional costs

vii Product reworks

viii Product quality impacts

ix Impacts on customer relations such as late delivery and lost of sales

x Cost of investigations into problem

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source with different type of loads

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

6

13 Project Objectives

The objectives of this project are

i To investigate suitable mitigation techniques for different type of voltage

sags source that connected to linear and non-linear load

ii To simulate and analyze the techniques using PSCADEMTDC software

iii To observe the effect on the characteristic of voltage sag such as the

magnitude and phase shift for each techniques

iv To make a few suggestions on the suitability of such techniques used for

both type of loads

14 Project Scope

The scopes for the project are

i Mitigation techniques that will be studied

a Dynamic Voltage Restorer (DVR)

b Distribution Static Compensator (D-STATCOM)

c Solid State Transfers Switch (SSTS) and

ii All techniques will be tested on different type of loads

iii Analysis will focus on effectiveness of each techniques in mitigating the

voltage sags

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

CHAPTER II

VOLTAGE SAGS

21 Introduction

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Tripping is caused by under voltage protection or over

current protection These two protections operate independently Large torque peaks

may cause damage to the shaft or equipment connected to the shaft Some common

reason for voltage sags are lightning strikes in power lines equipment failures

accidental contact power lines and electrical machine starts Despite being a short

duration between 10 milliseconds to 1 second event during which a reduction in the

RMS voltage magnitude takes place a small reduction in the system voltage can cause

serious consequences [5]

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

8

22 Definition of Voltage Sags

The definition of voltage sags is often set based on two parameters magnitude or

depth and duration However these parameters are interpreted differently by various

sources Other important parameters that describe voltage sags are

i the point-on-wave where the voltage sags occurs and

ii how the phase angle changes during the voltage sag A phase angle jump

during a fault is due to the change of the XR-ratio The phase angle jump

is a problem especially for power electronics using phase or zero-crossing

switching

The voltage sags as defined by IEEE Standard 1159 IEEE Recommended

Practice for Monitoring Electric Power Quality is ldquoa decrease in RMS voltage or current

at the power frequency for durations from 05 cycles to 1 minute reported as the

remaining voltagerdquo Typical values are between 01 pu and 09 pu and typical fault

clearing times range from three to thirty cycles depending on the fault current magnitude

and the type of over current detection and interruption [4]

Terminology used to describe the magnitude of voltage sag is often confusing

The recommended terminology according to IEEE Std 1159 is ldquothe sag to 20rdquo which

means that line voltage is reduced to 20 of normal value Another definition as given

in IEEE Std 1159 3173 is ldquoA variation of the RMS value of the voltage from nominal

voltage for a time greater than 05 cycles of the power frequency but less than or equal

to 1 minute Usually further described using a modifier indicating the magnitude of a

voltage variation (eg sag swell or interruption) and possibly a modifier indicating the

duration of the variation (eg instantaneous momentary or temporary)rdquo Figure 21

shows the rectangular depiction of the voltage sag

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

9

Figure 21 Depiction of voltage sag

23 Standards Associated with Voltage Sags

Standards associated with voltage sags are intended to be used as reference

documents describing single components and systems in a power system Both the

manufacturers and the buyers use these standards to meet better power quality

requirements Manufactures develop products meeting the requirements of a standard

and buyers demand from the manufactures that the product comply with the standard

[2]

The most common standards dealing with power quality are the ones issued by

IEEE IEC CBEMA and SEMI A brief description of each of the standards is provided

in next subtopic

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

10

231 IEEE Standard

The Technical Committees of the IEEE societies and the Standards Coordinating

Committees of IEEE Standards Board develop IEEE standards The IEEE standards

associated with voltage sags are given below [4]

IEEE 446-1995 ldquoIEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loadsrdquo

The standard discusses the effect of voltage sags on sensitive equipment motor

starting etc It shows principles and examples on how systems shall be designed to

avoid voltage sags and other power quality problems when backup system operates

IEEE 493-1990 ldquoRecommended practice for the design of reliable industrial and

commercial power systemsrdquo

The standard proposes different techniques to predict voltage sag characteristics

magnitude duration and frequency There are mainly three areas of interest for voltage

sags The different areas can be summarized as follows [4]

i Calculating voltage sag magnitude by calculating voltage drop at critical

load with knowledge of the network impedance fault impedance and

location of fault

ii By studying protection equipment and fault clearing time it is possible to

estimate the duration of the voltage sag

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

11

iii Based on reliable data for the neighborhood and knowledge of the system

parameters an estimation of frequency of occurrence can be made

IEEE 1100-1999 ldquoIEEE recommended practice for powering and grounding

electronic equipmentrdquo

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags It also explains the background and

application of the CBEMA (ITI) curves It is in some parts very similar to Std 1159 but

not as specific in defining different types of disturbances

IEEE 1159-1995 ldquoIEEE recommended practice for monitoring electric power

qualityrdquo

The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly It provides unique definitions for each type of

disturbance

IEEE 1250-1995 ldquoIEEE guide for service to equipment sensitive to momentary

voltage disturbancesrdquo

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion The primary purpose is to help identify

potential problems It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

12

equipment designer The second goal is to help designers of equipment to better

understand the environment in which their devices will operate The standard explains

different causes of sags lists of examples of sensitive loads and offers solutions to the

problems [4]

232 Industry Standard

2321 SEMI

The SEMI International Standards Program is a service offered by

Semiconductor Equipment and Materials International (SEMI) Its purpose is to provide

the semiconductor and flat panel display industries with standards and recommendations

to improve productivity and business SEMI standards are written documents in the form

of specifications guides test methods terminology and practices The standards are

voluntary technical agreements between equipment manufacturer and end-user The

standards ensure compatibility and interoperability of goods and services Considering

voltage sags two standards address the problem for the equipment [6]

SEMI F47-0200 ldquoSpecification for semiconductor processing equipment voltage

sag immunityrdquo

The standard addresses specifications for semiconductor processing equipment

voltage sag immunity It only specifies voltage sags with duration from 50ms up to 1s It

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

13

is also limited to phase-to-phase and phase-to-neutral voltage incidents and presents a

voltage-duration graph shown in Figure 22

SEMI F42-0999 ldquoTest method for semiconductor processing equipment voltage

sag immunityrdquo

This standard defines a test methodology used to determine the susceptibility of

semiconductor processing equipment and how to qualify it against the specifications It

further describes test apparatus test set-up test procedure to determine the susceptibility

of semiconductor processing equipment and finally how to report and interpret the

results [6]

Figure 22 Immunity curve for semiconductor manufacturing equipment according

to SEMI F47 [6]

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

14

2322 CBEMA (ITI) Curve

Information Technology Industry (ITI formally known as the Computer amp

Business Equipment Manufactures Association CBEMA) is an organization with

members in the IT industry Within the organization the Technical Committee 3 (TC3)

has published the ldquoITI (CBEMA) curve application noterdquo [7] The note describes an AC

input voltage that typically can be tolerated by most information technology equipment

The note is not intended to be a design specification (although it is often used by many

designers for that purpose) but a description of behavior for most IT equipment The

curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single-

phase information technology equipment [IEEE 1100 ndash 1999]

The voltage-time curve in Figure 23 describes the border of an area Above the

border the equipment shall work properly and below it shall shutdown in a controlled

way

Figure 23 Revised CBEMA curve ITIC curve 1996 [7]

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

15

This chapter has described the term ldquovoltage sagsrdquo and provided a foundation for

the following chapters The definitions provided by IEEE standards are the ones that are

used universally The characterization of voltage sags has also been discussed This

complies with the industry concerns related to the problem of power quality

24 General Causes and Effects of Voltage Sags

There are various causes of voltage sags in a power system Voltage sags can

caused by faults (more than 70 are weather related such as lightning) on the

transmission or distribution system or by switching of loads with large amounts of initial

starting or inrush current such as motors transformers and large dc power supply [3]

241 Voltage Sags due to Faults

Voltage sags due to faults can be critical to the operation of a power plant and

hence are of major concern Depending on the nature of the fault such as symmetrical or

unsymmetrical the magnitudes of voltage sags can be equal in each phase or unequal

respectively

For a fault in the transmission system customers do not experience interruption

since transmission systems are looped or networked Figure 24 shows voltage sag on all

three phases due to a cleared line-ground fault

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

16

Figure 24 Voltage sag due to a cleared line-ground fault

Factors affecting the sag magnitude due to faults at a certain point in the system

are

i Distance to the fault

ii Fault impedance

iii Type of fault

iv Pre-sag voltage level

v System configuration

a System impedance

b Transformer connections

The type of protective device used determines sag duration

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

17

242 Voltage Sags due to Motor Starting

Since induction motors are balanced 3 phase loads voltage sags due to their

starting are symmetrical Each phase draws approximately the same in-rush current The

magnitude of voltage sag depends on

i Characteristics of the induction motor

ii Strength of the system at the point where motor is connected

Figure 25 represents the shape of the voltage sag on the three phases (A B and

C) due to voltage sags

Figure 25 Voltage sag due to motor starting

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

18

243 Voltage Sags due to Transformer Energizing

The causes for voltage sags due to transformer energizing are

i Normal system operation which includes manual energizing of a

transformer

ii Reclosing actions

Figure 26 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature often depicted as a sudden drop in

system voltage followed by a slow recovery The main reason for transformer energizing

is the over-fluxing of the transformer core which leads to saturation Sometimes for

long duration voltage sags more transformers are driven into saturation This is called

Sympathetic Interaction Figure 26 show the voltage sag due to transformer energizing

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

CHAPTER III

PSCADEMTDC SOFTWARE

31 Introduction

In this project all the mitigation technique PSCADEMTDC software will be

used to simulate and analyze the techniques Power System Aided Design (PSCAD) was

first conceptualized in 1988 and began its evolution as a tool to generate data files for

the Electromagnetic Transient Program with DC Analysis (EMTDC) simulation

program In its early form Version was largely experimental Nevertheless it

represented a great leap forward in speed and productivity since users of EMTDC could

now draw their systems rather than creating text listings PSCAD was first introduced as

a commercial product as Version 2 targeted for UNIX platform in 1994 Version 3

comes in 1994 bringing new usability by fully integrating the drafting and runtime

systems of its predecessors This integration produced an intuitive environment for both

design and simulation [15]

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

20

PSCAD Version 4 represents the latest developments in power system simulation

software With much of the simulation engine being fully mature form many years the

new challenges lie in the advancement of the design tools for the user Version 4 retains

the strong simulation models of it predecessors while bringing the table an updated and

fresh new look and feel to its windowing and plotting

32 Characteristics of Software

PSCAD is a powerful and flexible graphical user interface to the world-

renowned EMTDC solution engine PSCAD enables the user to schematically construct

a circuit run a simulation analyze the results and manage the data in a completely

integrated graphical environment Online plotting function controls and meters are also

included so that the user can alter system parameters during a simulation run and view

the results directly [15]

PSCAD comes complete with a library of pre-programmed and tested models

ranging from simple passive elements and control functions to more complex models

such as electric machines FACTS devices transmission lines and cables If a particular

model does not exist PSCAD provides the flexibility of building custom models either

by assembling them graphically using existing models or by utilizing an intuitively

Design Editor

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

21

The following are some common models found in systems studied using

PSCAD

i Resistors inductors capacitors

ii Mutually coupled windings such as transformers

iii Frequency dependent transmission lines and cables (including the most

accurate time domain line model in the world)

iv Current and voltage sources

v Switches and breakers

vi Protection and relaying

vii Diodes thyristors and GTOs

viii Analog and digital control functions

ix AC and DC machines exciters governors stabilizers and initial models

x Meters and measuring functions

xi Generic DC and AC controls

xii HVDC SVC and other FACTS controllers

xiii Wind source turbine and governors

PSCAD Version 4 has some major features that have been included prior to its

predecessors for usersrsquo convenience in modeling and analysis of custom power system

such as

i Windowing Interface ndash PSCAD V4 boasts a completely new windowing

interface which includes full MFC (Microsoft Foundation Class)

compatibility docking window support and a new integrated design

editor

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

22

ii Drawing Interface ndash the drawing interface has been enhanced to provide

uniform messaging and core support as well as a full double-buffered

display

iii On-Line Plotting Tools ndash the online plotting facilities in PSCAD V4 have

been completely redesigned and are now more powerful The new

advanced graphs come complete with full features including full zoom

and panning support marker control Polymeter and XY plotting

capabilities

iv Off-Line Plotting Facilities ndash with the inclusion of Livewire the best data

visualization and analysis software package available today PSCAD

output come to life

v Single-Line Diagram Input ndash PSCAD now includes the ability to

construct a circuits in a convenient and space saving single-line format

This new feature includes fully adaptive three-phase electrical

components in the Master Library can be adjusted easily to display a

single-line equivalent view

vi MATLABregSIMULINKreg Interface ndash now interface PSCAD to both

MATLABreg andor SIMULINKreg files

33 Example of Circuit

A typical DVR built in PSCAD and installed into a simple power system to

protect a sensitive load in a large radial distribution system [4] is presented in Figure 31

The coupling transformer with either a delta or wye connection on the DVR side is

installed on the line in front of the protected load Filters can be installed at the coupling

transformer to block high frequency harmonics caused by DC to AC conversion to

reduce distortion in the output The DC voltage source is an external source supplying

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

23

DC voltage to the inverter to convert to AC voltage The optimization of the DC source

can be determined during simulation with various scenarios of control schemes DVR

configurations performance requirements and voltage sags experienced at the point

DVR is installed

Figure 31 DVR with main components in PSCAD

The inverter is a six-pulse gate turn off (GTO) thyristor controlled bridge

Currents will follow in different directions at outputs depending on the control scheme

eventually supplying AC output power to the critical load during power disturbances

The control of this bridge is indeed the control of thyristor firing angles Time to open

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

24

and close gates will be determined by the control system There are several methods for

controlling the inverter To model a DVR protecting a sensitive load against only

balanced voltage sags a simple method of using the measurement of three-phase rms

output voltage for controlling signals can be applied Amplitude modulation (AM) is

then used In addition to provide appropriate firing angles to thyristor gates the

switching control using pulse width modulation (PWM) technique and interpolation

firing is employed

Figure 32 The Wye-Connected DVR in PSCAD

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

25

In Figure 32 the transformer is wye-connected with a common connection to the

midpoint of the DC source This allows that current will pump into each phase through

each pair of GTO and then return without affecting the other two phases It is noted that

to maintain an equal injecting voltage to each phase the same value of DC voltage at

each half of the source would be required

34 Conclusion

PSCAD Version 4 is a powerful tools to simulate and analysis custom power

systems With all the benefits designing a systems is as simple as using a drawing board

and a pencil in our hands Many new models have been added to the PSCAD Master

Library since the last release of PSCAD V3 thus improving capability of designing

Navigating the software is now has been made easy with the multi-window tab feature

and toolbars Common components were made available and easy to drag-and-drop it to

the drawing board

All those features were shadowed over with the limitation due to its commercial

value It has been described in the manual as Dimension Limits Those limits are divided

into two major groups which are Edition Specific Limits and Compiler Specific Limits

As for this project those limitations be of less interest because only one subsystem that

will be analysis for each mitigation technique

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

CHAPTER IV

VOLTAGE SAG MITIGATION TECHNIQUES

41 Introduction

Different power quality problems would require different solution It would be

very costly to decide on mitigate measure that do not or partially solve the problem

These costs include lost productivity labor costs for clean up and restart damaged

product reduced product quality delays in delivery and reduced customer satisfaction

Voltage sag can be classified in power quality problem Hence when a customer

or installation suffers from voltage sag there is a number of mitigation methods are

available to solve the problem These responsibilities are divided to three parts that

involves utility customer and equipment manufacturer Figure 41 shows the different

protection options for improving performance during power quality variation [1]

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

27

Figure 41 Different protection options for improving performance during power

quality variation [1]

This project intends to investigate mitigation technique that is suitable for

different type of voltage sags source with different type of loads The simulation will be

using PSCADEMTDC software The mitigation techniques that will be studied such as

using dynamic voltage restorer (DVR) distribution static compensator (DSTATCOM)

and solid state transfer switch (SSTS)

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

28

42 Dynamic Voltage Restorer (DVR)

Voltage magnitude is one of the major factors that determine the quality of

power supply Loads at distribution level are usually subject to frequent voltage sags due

to various reasons Voltage sags are highly undesirable for some sensitive loads

especially in high-tech industries It is a challenging task to correct the voltage sag so

that the desired load voltage magnitude can be maintained during the voltage

disturbances [8]

The effect of voltage sag can be very expensive for the customer because it may

lead to production downtime and damage Voltage sag can be mitigated by voltage and

power injections into the distribution system using power electronics based devices

which are also known as custom power device [9] Different approaches have been

proposed to limit the cost causes by voltage sag One approach to address the voltage

sag problem is dynamic voltage restorer (DVR) It can be used to correct the voltage sag

at distribution level

441 Principles of DVR Operation

A DVR is a solid state power electronics switching device consisting of either

GTO or IGBT a capacitor bank as an energy storage device and injection transformers

It is connected in series between a distribution system and a load that shown in Figure

42 The basic idea of the DVR is to inject a controlled voltage generated by a forced

commuted converter in a series to the bus voltage by means of an injecting transformer

A DC capacitor bank which acts as an energy storage device provides a regulated dc

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

29

voltage source A DC to Ac inverter regulates this voltage by sinusoidal PWM

technique

During normal operating condition the DVR injects only a small voltage to

compensate for the voltage drop of the injection transformer and device losses

However when voltage sag occurs in the distribution system the DVR control system

calculates and synthesizes the voltage required to maintain output voltage to the load by

injecting a controlled voltage with a certain magnitude and phase angle into the

distribution system to the critical load [9]

Figure 42 Principle of DVR with a response time of less than one millisecond

Note that the DVR capable of generating or absorbing reactive power but the

active power injection of the device must be provided by an external energy source or

energy storage system The response time of DVD is very short and is limited by the

power electronics devices and the voltage sag detection time The expected response

time is about 25 milliseconds and which is much less than some of the traditional

methods of voltage correction such as tap-changing transformers [8]

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

30

43 Distribution Static Compensator (DSTATCOM)

In its most basic function the DSTATCOM configuration consist of a two level

voltage source converter (VSC) a dc energy storage device a coupling transformer

connected in shunt with the ac system and associated control circuit [10 11] as shown

in Figure 43 More sophisticated configurations use multipulse andor multilevel

configurations as discussed in [12] The VSC converts the dc voltage across the storage

device into a set of three phase ac output voltages These voltages are in phase and

coupled with the ac system through the reactance of the coupling transformer Suitable

adjustment of the phase and magnitude of the DSTATCOM output voltages allows

effective control of active and reactive power exchanges between the DSTATCOM and

the ac system

Figure 43 Schematic diagram of the DSTATCOM as a custom power controller

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

31

The VSC connected in shunt with the ac system provides a multifunctional

topology which can be used for up to three quite distinct purposes [13]

i Voltage regulation and compensation of reactive power

ii Correction of power factor

iii Elimination of current harmonics

The design approach of the control system determines the priorities and functions

developed in each case In this case DSTATCOM is used to regulate voltage at the point

of connection The control is based on sinusoidal PWM and only requires the

measurement of the rms voltage at the load point

441 Basic Configuration and Function of DSTATCOM

The DSTATCOM is a three phase and shunt connected power electronics based device

It is connected near the load at the distribution systems The major components of the

DSTATCOM are shown in Figure 44 below It consists of a dc capacitor three phase

inverter module such as IGBT or thyristor ac filter coupling transformer and a control

strategy The basic electronic block of the DSTATCOM is the voltage sourced converter

that converts an input dc voltage into three phase output voltage at fundamental

frequency

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

32

Figure 44 Building blocks of DSTATCOM

Referring to Figure 44 the controller of the DSTATCOM is used to operate the

inverter in such a way that the phase angle between the inverter voltage and the line

voltage is dynamically adjusted so that the DSTATCOM generates or absorbs the

desired VAR at the point of connection The phase of the output voltage of the thyristor

based converter Vi is controlled in the same way as the distribution system voltage Vs

Figure 45 shows the three basic operation modes of the DSTATCOM output current I

which varies depending upon Vi

For instance if Vi is equal to Vs the reactive power is zero and the DSTATCOM

does not generate or absorb reactive power When Vi is greater than Vs the

DSTATCOM lsquoseesrsquo an inductive reactance connected at its terminal Hence the system

lsquoseesrsquo the DSTATCOM as a capacitive reactance The current I flows through the

transformer reactance from the DSTATCOM to the ac system and the device generates

capacitive reactive power Furthermore if Vs is greater than Vi the system lsquoseesrsquo and

inductive reactance connected at its terminal and the DSTATCOM lsquoseesrsquo the system as a

capacitive reactance then the current flows from the ac system to the DSTATCOM

resulting in the device absorbing inductive reactive power

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

33

Figure 45 Operation modes of a DSTATCOM

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

34

44 Solid State Transfer Switch (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage

sags swells and other electrical disturbance [14] The SSTS ensures continuous high

quality power supply to sensitive loads by transferring within a time scale of

milliseconds the load from a faulted bus to a healthy one

The basic configuration of this device consists of two three phase solid state

switches one for main feeder and one for the backup feeder These switches have an

arrangement of back-to-back connected thyristors as illustrated in Figure 46

Figure 46 Schematic representations of the SSTS as a custom power device

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

35

Each time a fault condition is detected in the main feeder the control system

swaps the firing signals to the thyristor in both switches in example Switch 1 in the

main feeder is deactivated and Switch 2 in the backup feeder is activated The control

system measures the peak value of the voltage waveform at every half cycle and checks

whether or not it is within a prespecified range If it is outside limits an abnormal

condition is detected and the firing signals of the thyristors are changed to transfer the

load to the healthy feeder

441 Basic Configuration and Function of SSTS

The SSTS as shown in Figure 47 is a high speed open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few

milliseconds

Figure 47 Solid State Transfer Switch system

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

36

The open-transition property of the SSTS means that the switch break contact

with one source before it makes contact with the other source The advantage of this

transfer scheme over the closed-transition mechanical switch is that the electrical

sources are never cross-connected unintentionally The cross connection of independent

ac sources with the alternate source switching on to a faulted system is discouraged by

electric utilities

The solid state transfer switch consists of two three phase ac thyristor switches

The thyristor operating in its two modes forms the key component of the SSTS In the

ON-state mode low impedance forward conduction of current takes place In the OFF-

state mode an open circuit with almost infinite impedance occurs in the thyristor

The basic ON-state and OFF-state properties of the thyristor are used to form an

intelligent switch which can choose between two upstream power sources providing the

better quality of supply available to the electrical load downstream The basic

configuration is based on anti-parallel thyristor group on preferred and alternate sides of

the switch A thyristor allows conduction only in forward direction Figure 48 illustrate

how the thyristors of transfer switch 1 can conduct either in the positive or the negative

half cycle of the ac sinusoid and the supply path is indicated by the bold line

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

37

Figure 48 Thyristors of the SSTS conducting in the positive and negative half cycle

of the preferred source

During normal operation thyristors associated with the preferred source are in

the ON-state normally closed (NC) position while those associated with the alternate

source are in the OFF-state normally open (NO) position

Current sensing circuits constantly monitor the states of the preferred and

alternate sources and feed the information to the monitoring high speed controller Upon

detecting the loss of the preferred source or voltage that is not within the preset range

the controller blocks the firing impulse signals to the gate-driven thyristors of transfer

switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safe

interlocking mechanism Power then flows via the path as indicated by the bold line in

Figure 49

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

38

Figure 49 Thyristors on the alternate supply are turned ON on a sensing a

disturbance on the preferred source

The mechanical bypass equipment provides conventional transfer switch

functionality when the SSTS is in a thermal overload condition or is out of service for

testing or maintenance

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

CHAPTER V

MITIGATION TECNIQUES REALIZATION

51 Sinusoidal PWM-Based Control Scheme

In order to mitigate the simulated voltage sags in the test system of each

mitigation technique also to mitigate voltage sags in practical application a sinusoidal

PWM-based control scheme is implemented with reference to the DSTATCOM The

control scheme for the DVR follows the same principle The aim of the control scheme

is to maintain a constant voltage magnitude at the point where sensitive load is

connected under the system disturbance

The control system only measures the rms voltage at load point [10] in example

no reactive power measurements is required [17] The VSC switching strategy is based

on a sinusoidal PWM technique which offers simplicity and good response Since

custom power is a relatively low-power application PWM methods offer a more flexible

option than the fundamental frequency switching (FFS) methods favored in FACTS

applications Besides high switching frequencies can be used to improve the efficiency

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

40

of the converter without incurring significant switching losses Figure 51 shows the

DSTATCOM controller scheme implemented in PSCADEMTDC The DSTATCOM

control system exerts voltage angle control as follows an error signal is obtained by

comparing the reference voltage with the rms voltage measured at the load point The PI

controller processes the error signal and generates the required angle δ to drive the error

to zero in example the load rms voltage is brought back to the reference voltage In the

PWM generators the sinusoidal signal vcontrol is phase modulated by means of the angle

δ or delta as nominated in the Figure 51 The modulated signal vcontrol is compared

against a triangular signal (carrier) in order to generate the switching signals of the VSC

valves

Figure 51 Control scheme for the test system implemented in PSCADEMTDC to

carry out the DSTATCOM and DVR simulations

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

41

The main parameters of the sinusoidal PWM scheme are the amplitude

modulation index ma of signal vcontrol and the frequency modulation index mf of the

triangular signal The vcontrol in the Figure 51 are nominated as CtrlA CtrlB and CtrlC

The amplitude index ma is kept fixed at 1 pu in order to obtain the highest fundamental

voltage component at the controller output [13 18] The switching frequency mf is set at

450 Hz mf = 9 It should be noted that an assumption of balanced network and

operating conditions are made

The modulating angle δ or delta is applied to the PWM generators in phase A

whereas the angles for phase B and C are shifted by 240deg or -120deg and 120deg respectively

It can be seen in Figure 51 that the control implementation is kept very simple by using

only voltage measurements as feedback variable in the control scheme The speed of

response and robustness of the control scheme are clearly shown in the test results

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

42

52 Test System

Figure 52 The test system implemented in PSCADEMTDC

Figure 52 depict the test system implemented in PSCADEMTDC to carry out

the simulations for the aforementioned mitigation techniques The test system comprises

of a 230 kilovolt 50 Hertz transmission system represented in Thevenin equivalent

feeding into the primary side of a 2-winding transformer The load is connected to the 11

kilovolt secondary side of the transformer Another 3-winding transformer will be used

to replace the 2-winding transformer to accommodate the implantation of the two-level

DSTATCOM and it will be connected in the tertiary winding of the transformer to

provide instantaneous voltage support at the load point The transformer employ a

leakage reactance of 10 or 01 per unit with a unity turns ratio and no booster

capabilities exist

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

43

53 Dynamic Voltage Restorer

The DVR is a powerful controller that is commonly used for voltage sags

mitigation at the point of connection The DVR employs the same block as the

DSTATCOM but in this application the coupling transformer is connected in series with

the ac system as illustrated in Figure 53 The VSC generates a three-phase ac output

voltage which is controllable in phase and magnitude These voltages are injected into

the ac system in order to maintain the load voltage at the desired voltage reference The

main features of the DVR control scheme have been explained in section 51

Figure 53 One line diagram of the DVR test system

The DVR that have been used to test the system in section 51 is shown in Figure

54 The DVR is basically the same as DSTATCOM but instead of using a capacitor

DVR employs 5 kilovolt dc storage supply The DVR is then connected in series using

transformers in delta to the lines Figure 55 will show the full test system to realize the

effectiveness of the DVR control

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

44

Figure 54 Schematic diagram of the DVR

Figure 55 Schematic diagram of the test system with DVR connected to the system

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

45

54 Distribution Static Compensator

The test system employed to carry out the simulations concerning the

DSTATCOM actuation is shown in Figure 29 which is the same system presented in

[16] A two-level DSTATCOM is connected to the 11 kV tertiary winding to provide

instantaneous voltage support at the load point A 750 microF capacitor on the dc side

provides the DSTATCOM energy storage capabilities

The transformer of the test system has been changed to a 3-winding transformer

to accommodate DSTATCOM The purpose of including the transformer is to protect

and provide isolation between the IGBT legs This prevents the dc storage capacitor

from being shorted through switches in different IGBT Figure 56 shows the build of

the DSTATCOM in PSCADEMTDC which is the two-level voltage source converter

and the realization of the test system being employed shown in Figure 57

Figure 56 One line diagram of the DSTATCOM test system

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

46

Figure 57 Schematic diagram of the test system with DSTATCOM connected to the

system

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

47

55 Solid State Transfer Switch

In the test to carry out the SSTS simulations the system comprises with two

identical feeders from section 51 and a sensitive load connected to the bus bar Figure

58 shows the system that is employed

Figure 58 One line diagram of the SSTS test system

Simulations were carried out to assess the effectiveness of the simple control

scheme that has been employed in the system proposed earlier Figure 59 shows the

SSTS system that being employed for the test in PSCADEMTDC It comprises of two

sets of switches which is switch group 1 and switch group 2 that alternately turns ON

and OFF corresponds to the fault detector signals The full system application to test the

SSTS is shown in Figure 510

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

48

Figure 59 SSTS switches implemented in PSCADEMTDC

Figure 510 Schematic diagram of the test system with SSTS connected to the system

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

CHAPTER VI

SIMULATIONS AND RESULTS

61 Test case

This section contains the results of the simulations to assess the capability of

each technique to mitigate various fault sources In order to make a fair assessment the

simulations only use one test system as proposed in section 51 The test were divide into

the most common faults which are

611 Single line to ground fault and

612 Double line to ground fault

The most common fault is the single line to ground faults which covers 70 of

total faults There are many situations that can make the occurrence of single line to

ground faults possible The low impedance faults are referred to as bolted faults

indicating that the faulted conductors are effectively bolted together to create a line to

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

50

line faults which cover 10 of the total faults or double line to fault for the total of 15

A much more common effect is where the fault has some finite impedance When a line

falls on sandy soil or there is a significant distance for an arc to jump then the

characteristic may have a constant voltage characteristic The remaining 5 of the faults

are three phase faults

62 Single line to ground fault

621 Phase A to ground

Using the faults generator Figure 61a clearly shows a phase shift of line A after

the fault has been applied The angle of the line shifted as much as 8844deg from the

reference angle for line A of -194deg For the rms value of the line we can refer to Figure

61b which clearly shows the voltage sag The value of the rms has been normalized and

for the phase A to the ground fault the rms drops to 0685 or nearly 31 from the

reference value

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

51

(a)

(b)

Figure 61 (a) Phase shift for line A to the ground fault (b) Rms voltage drop

The simulations have two parts which have been run separately This first part

involves simulating the test system on different fault as mention above The second part

involves simulating the mitigation techniques with the test system so that each of the

technique can be assessed on their performance in mitigating voltage sags

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

52

(a)

(b)

Figure 62 (a) Corrected phase with DVR (b) Compensated voltage sag with DVR

The first technique that has been used is the DVR Figure 62a shows the

capability of the technique to balance the phase shift while Figure 62b shows how the

technique compensates the voltage drop DVR recover almost 96 of the reference

voltage

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

53

The second technique that has been used in mitigating the voltage sags and phase

shift is the DSTATCOM Figure 63a shows the phase balance of the system and Figure

63b shows the recovery of the voltage sags DSTATCOM manage to recover nearly

94 of the voltage with respect to the reference voltage

(a)

(b)

Figure 63 (a) Corrected phase using DSTATCOM (b) Compensated voltage sag

using DSTATCOM

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

54

The third technique that has been used is SSTS In SSTS whenever the fault

detector control scheme detects a faulty line it changes the firing angle of the switches

that are connected to the line thus change the feed from the main feeder to the alternative

or backup feed Figure 64a and Figure 64b clearly shows that no interruption can be

noticed since the backup feeder is healthy

(a)

(b)

Figure 64 (a) Corrected phase using SSTS (b) Compensated voltage sag using

SSTS

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

55

Since SSTS switch the faulty feeder with the healthy one whenever faults occur

as long as the back up feeder is healthy the result produced by this technique will

always be the same Hence the result of the SSTS will be omitted hereafter with the

assumption that the backup feeder is always healthy

Table 61 (a) Test results for line A to the ground fault (b) Recovery result

TEST 1 PHASE A TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12194 11806 0685 0991

DVR 075 -9893 9832 0923 0963

DSTATCOM 128 -14787 1424 0948 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 1 PHASE A TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 8963 2301 1974 9585

DSTATCOM 891 2593 2434 9377

SSTS 8849 005 005 100

(b)

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

56

From table 61a and 61b we can see that SSTS has the best recovery rate since it

doesnrsquot involve compensating technique either to absorb or inject power to the system

The rms value of the system is always constant It is different than the other two

techniques which require them to inject or absorb power to and from the system DVR

has better recovery in mitigating the voltage sag than DSTATCOM but poor in

correcting the phase of the lines DVR recover 2 better in comparison with

DSTATCOM

622 Phase B to ground

For test 2 the faults generator still emulates a single line to ground fault of line

B it is applied from 25 milliseconds to 35 milliseconds The rms value of the faulty

system is as the same as Figure 61b The only difference is in the phase of the system

Figure 65 show the shifted phase of the system when the fault occurs

Figure 65 Phase shift of line B to the ground fault

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

57

It can be noticed that phase B has been shifted 90deg to 150deg for the duration of the

fault Figure 66a shows the result from DVR mitigation and Figure 66b shows the

result for DSTATCOM for phase correction Each technique recovers the same value of

the rms as when it mitigates the phase A to the ground fault

(a)

(b)

Figure 66 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B to the ground fault

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

58

From the figure above it can be observed that other line phases were also

affected when both techniques try to correct the lines phase The effect can be clearly

noted in Figure 66a where the phase of line A and C are shifted even though those lines

were not in fault This condition as well happen when DSTATCOM try to correct the

phases The result of the test is shown in Table 62(a) whereas Table 62(b) will show

the recoveries that have been achieved by those three techniques

Table 62 (a) Test results for line B to the ground fault (b) Recovery result

TEST 2 PHASE B TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 14964 11806 0686 0991

DVR -21 -11856 140 0923 0963

DSTATCOM 1583 -12237 9672 0942 1016

SSTS -189 -12189 11811 0989 0989

(a)

TEST 2 PHASE B TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1906 3108 2194 9585

DSTATCOM 1389 2727 2134 9272

SSTS 005 2775 005 100

(b)

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

59

DVR manage to recover 9585 of the rms voltage with respect to the reference

value and DSTATCOM recover 3 less of DVR For SSTS the recovery rate is always

100 since the backup feeder is healthy

623 Phase C to ground

Test 3 involves line C of the system This test is practically the same as previous

test which only involves 1 line of the system The results of the rms voltage is the same

as Figure 61(b) but the phase of line C is shifted as much as 90deg and can be seen in

Figure 67

Figure 67 Phase shift of line B to the ground fault

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

60

Mitigation of the fault outcome is the same product as the preceding test which

DVR and DSTATCOM compensate the rms voltage similarly Figure 68(a) and Figure

68(b) shows the phase difference for the mitigation technique accordingly

(a)

(b)

Figure 68 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line C to the ground fault

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

61

The numerical result will be shown in Table 63(a) whereas the recovery will be

shown in Table 63(b) The phase of line C has been corrected but at the same time

other lines were also affected This is true for both of the technique but not for SSTS

which is the same as Figure 64(a) and Figure 64(b)

Table 63 (a) Test results for line C to the ground fault (b) Recovery result

TEST 3 PHASE C TO GROUND

PHASE(deg) RMS(pu) TECHNIQUES

A B C min max

FAULT -194 -12194 2969 0686 0991

DVR 1969 -13945 11742 0923 0963

DSTATCOM -2283 -10183 12867 0914 1011

SSTS -189 -12189 11811 0989 0989

(a)

TEST 3 PHASE C TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 1775 1751 8773 9585

DSTATCOM 2089 2011 9898 9041

SSTS 005 005 8842 100

(b)

From the table line A and line B should have stay fixed on 0deg and -120deg

respectively but after DVR and DSTATCOM try to correct the phase of line C the

phase of those lines were shifted to 20deg and -149deg for DVR and -23deg and -102deg for

DSTATCOM This could be due to the control scheme that is too simple In the mean

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

62

time the rms voltage compensation for both DVR and DSTATCOM are still above 90

in respect to the reference voltage DVR still maintain plusmn5 from the overall voltage

This is true for the entire tests that have been carried out before while SSTS results are

overwhelming with no ripple or overshoot

63 Double lines to ground fault

The next line of test is double line to the ground fault As an overall those

techniques except SSTS suffer terrible loss when its try to mitigate double line to the

ground fault This fault only covers 15 of overall fault that occurs practically but it

pose much more danger to the loads that draw supply from the lines

631 Phase A and B to ground

The first test to come is line A and line B to the ground fault The effect of this

fault is depicted in Figure 68(a) which shows the phase fault and Figure 68(b) that

shows the rms voltage of the test system during the fault

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

63

(a)

(b)

Figure 69 (a) Phase shift for line A and B to the ground fault (b) Rms voltage drop

For this test the phase A and B has been shifted 90deg to -90deg and 150deg

respectively The voltage drop is doubled from previous test set to 0366 per unit with

respect to the reference voltage Figure 610(a) shows the result of the DVR try to

correct the shifted phases for the fault and Figure 610(b) shows for the DSTATCOM

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

64

(a)

(b)

Figure 610 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and B to the ground fault

As we can see from the figure DVR continue to correct the phases of the faulted

lines steadily with almost the same value at the time DVR is correcting the single line to

ground fault The same abnormality happens with the line that doesnrsquot need any

correction and in this case it is line C The phase of line C is shifted nearly 10deg

However DSTATCOM capability of correcting the phase of single line to the ground

fault has not been continual for the double line to the ground fault For lines A and B to

the ground fault DSTATCOM is able to correct the phase of line B but this is not

occurred to line A The phase is shifted about 140deg and rest at 50deg

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

65

Even though the voltage sag is double from the previous value DVR manage to

compensate the voltage drop and recovered nearly 90 with respect to the reference

voltage DSTATCOM only manage to recover 78 This is due to the inability of

DSTATCOM to mitigate double line to the ground fault with only using simple control

scheme that has been introduced in section 51 It is clearly shown in Figure 611(a) and

611(b) for DVR and DSTATCOM respectively

(a)

(b)

Figure 611 (a) Compensated voltage sag using DVR (b) Compensated voltage sag

using DSTATCOM Line A and B to the ground fault

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

66

The value of voltage sag that have been recovered for other double lines to the

ground fault such as line A and C to the ground fault and line B and C to the ground

fault is the same as the result shown in Figure 611 Hence those results are omitted

hereafter

Table 64(a) will show the full result of line A and B to the ground fault while

Table 64(b) shows the recovered voltage sag and corrected phase for those lines

Table 64 (a) Test results for line A and B to the ground fault (b) Recovery result

TEST 4 PHASE AB TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 14966 11806 0366 0991

DVR -078 -1106 110331 0858 0963

DSTATCOM 4961 -12336 11725 0777 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 4 PHASE AB TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 896 3906 7729 891

DSTATCOM 4077 263 081 7841

SSTS 8849 2777 005 100

(b)

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

67

632 Phase A and C to ground

The next test case is line A and C to the ground fault As mention before the

result of voltage sag that is mitigated is the same as the result for section 631 DVR and

DSTATCOM recover the same value as its try to mitigate test case 4 Therefore the

results of voltage sag mitigation of this section are omitted

Figure 612 Phase shift for line A and C to the ground fault

Figure 612 shows the phases that are in fault The phase of line A is shifted 90deg

to rest at -90deg while the phase of line C is also shifted 90deg and stays at 30deg during the

fault The result of the corrected phase will be shown in Figure 613(a) and 613(b) for

DVR and DSTATCOM respectively

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

68

(a)

(b)

Figure 613 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line A and C to the ground fault

The result in Figure 613(b) clearly shows the improper phase correction of line

C which definitely affect the result of DSTATCOM voltage mitigation while in Figure

613(a) DVR also cannot correct the phase accurately The full test result is shown in

Table 65(a) while Table 65(b) shows the recovery result

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

69

Table 65 (a) Test results for line A and C to the ground fault (b) Recovery result

TEST 5 PHASE AC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -9038 -12193 2965 0365 0991

DVR -1982 -11938 1393 0858 0963

DSTATCOM 286 -12898 17872 0769 0995

SSTS -189 -12189 11811 0989 0989

(a)

TEST 5 PHASE AC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 7056 255 10965 891

DSTATCOM 8752 705 14907 7729

SSTS 8849 004 8846 100

(b)

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

70

633 Phase B and C to ground

The last test case is line B and C to the ground fault In this case phase B is

shifted 90deg to end at 150deg and phase C is also shifted 90deg and stays at 30deg respectively

This can be seen in Figure 614 as it shows the phase shift of the faulty lines

Figure 614 Phase shift for line B and C to the ground fault

The phase of line A is unaffected by the fault of other lines throughout the fault

period However the phase of the line is affected and shifted 30deg for the moment of

mitigation using DVR This affect is obviously depicted in Figure 615(a)

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

71

(a)

(b)

Figure 615 (a) Phase correction using DVR (b) Phase correction using DSTATCOM

line B and C to the ground fault

As typically happened for DSTATCOM one of the faulty lines in Figure 615(b)

is not corrected appropriately and this time it is line B The phase of the line at the time

of mitigation is -60deg as it suppose to be at -120deg The full result of the test is shown in

Table 66(a) and the recovery result is shown in Table 66(b)

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

72

Table 66 (a) Test results for line B and C to the ground fault (b) Recovery result

TEST 6 PHASE BC TO GROUND

PHASE(deg) VRMS(pu) TECHNIQUES

A B C min max

FAULT -193 14965 2968 0365 0991

DVR 3073 -13593 14793 0858 0963

DSTATCOM -626 -616 12603 0768 0991

SSTS -189 -12189 11811 0989 0989

(a)

TEST 6 PHASE BC TO GROUND RECOVERY

PHASE(deg) VRMS() TECHNIQUES

A B C GAIN

DVR 288 1372 11825 891

DSTATCOM 433 8805 9635 775

SSTS 004 2776 8843 100

(b)

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

73

64 Conclusion

In mitigating single line to the ground fault DVR and DSTATCOM that has

been introduced in section 5 are able to compensate the voltage sag without any

difficulty The problem lies in correcting the phase of the system Even though the phase

of the faulty line has been corrected the rest of the lines that are not in fault is also

affected and shifted a few degrees This affect can be seen happened to DVR when it

mitigates the test system In general the capability of the techniques to mitigate single

line to the ground fault are uncontested especially SSTS as it pose the best result

While mitigating double lines to the ground fault the same problems occurred to

the DVR where the phase of the healthy line is unwontedly shifted a few degrees but the

performance of DVR in mitigating voltage sag remain the same as it mitigates single

line to the ground fault For DSTATCOM a new problem occurred while DSTATCOM

is mitigating double line to the ground fault One of the faulty lines is not corrected

appropriately and this brings an upsetting effect in mitigating the voltage sag of the

system Once again SSTS that has been introduced in section 5 remain as the best

mitigation technique This is due to the nature of the SSTS where it doesnrsquot try to

compensate or correct the faulty line instead SSTS switch the faulty feeder to the

alternative feeder The result is always and remains constant if and only if the backup or

alternative feeder is being kept healthy

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

CHAPTER VII

CONCLUSION

71 Conclusion

Nowadays reliability and quality of electric power is one of the most discuss

topics in power industry There are numerous types of power quality issues and power

problems and each of them might have varying and diverse causes The types of power

quality problems that a customer may encounter classified depending on how the voltage

waveform is being distorted There are transients short duration variations (sags swells

and interruption) long duration variations (sustained interruptions under voltages over

voltages) voltage imbalance waveform distortion (dc offset harmonics interharmonics

notching and noise) voltage fluctuations and power frequency variations Among them

two power quality problems have been identified to be of major concern to the

customers are voltage sags and harmonics but this project is focusing on voltage sags

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

75

Voltage sags are huge problems for many industries and it is probably the most

pressing power quality problem today Voltage sags may cause tripping and large torque

peaks in electrical machines Generally voltage sags are short duration reductions in rms

voltage caused by faults in the electric supply system and the starting of large loads

such as motors Voltage sags are also generally created on the electric system when

faults occur due to lightning which are accidental shorting of the phases by trees

animals birds human error such as digging underground lines or automobiles hitting

electric poles and failure of electrical equipment Sags also may be produced when large

motor loads are started or due to operation of certain types of electrical equipment such

as welders arc furnaces smelters etc

Therefore this project intends to investigate mitigation technique that is suitable

for different type of voltage sags source The simulation will be using PSCADEMTDC

software and the mitigation techniques that using such as dynamic voltage restorer

(DVR) distribution static compensator (DSTATCOM) and solid state transfer switch

(SSTS)

Dynamic voltage restorers (DVR) are used to protect sensitive loads from the

effects of voltage sags on the distribution feeder In all cases it is necessary for the DVR

control system to not only detect the start and end of a voltage sag but also to determine

the sag depth and any associated phase shift The DVR which is placed in series with a

sensitive load must be able to respond quickly to voltage sag if end users of sensitive

equipment are to experience no voltage sags

The distribution static compensator (DSTATCOM) offers an alternative to

conventional series shunt compensation In the traditional power transmission system

controllable devices are restricted to the slow mechanisms such as transformer tap

changers and switched capacitor In the late 1980rsquos thanks to the major developments

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

76

in the semiconductor technology it became possible to apply power electronics in the

control of DSTATCOM Based on the simulation therersquos a room for improvement

DSTATCOM is a device that promises a prominent feature in power system in

mitigating power quality related problems in the future

Solid state transfer switch (SSTS) is not the most cost effective but in many

cases it is a practical mitigating technique to apply especially for sensitive loads These

solutions involve fixing the two identical power source components in order to increase

the ride-through of the entire system SSTS solutions are attractive since they in theory

do not require add on power conditioning equipment but instead involve using another

source components Furthermore semiconductor tool suppliers are more comfortable

with this approach since it does not require the addition of unfamiliar technologies

As conclusion voltage sag is unwanted phenomenon which unavoidable but can

be reduced using all techniques but not limited to the techniques that have been

discussed There is no one mitigation technique that will suitable with every application

and whilst the power supply utilities strive to supply improved power quality it is up to

the applications engineer to minimize power quality problems It means power quality

problem cannot be eliminated but we can reduce and try to avoid this problem form

occur The best way to avoid power quality problem is by ensuring that all equipment to

be installed in the industrial plants are compatible with power quality in the power

system This can be achieved by procuring equipment with proper technical

specifications that incorporate power quality performance of its operating electrical

environment

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

77

72 Suggestion

Mitigating voltage sag requires a lot of intensive research especially in

developing custom power device to help distribution system to achieve desired power

quality as been insisted by many customer or end-user There are still rooms of

improvement that can be achieved further for the technique that have been included in

this thesis and other techniques that are available

The DVR and DSTATCOM that has been used earlier employs a two- level

voltage source converter or VSC in both technique Additional research of other

multilevel and multipulse VSC can be implemented in the future to exploit the simplicity

of the pulse width modulation or PWM based control scheme to further enhance both

DVR and DSTATCOM Another control scheme can also be proposed to take the

advantage of the two-level VSC that has been employed previously to support more

control over voltage sags that were caused by double line to ground line to line faults

and three phase fault that cover 25 percent of the total faults

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

78

REFERENCES

[1] Roger C Dugan Mark F McGranaghan and H Wayne Beaty

TK1001D84 (1996) ldquoElectrical Power Systems Qualityrdquo Mc Graw-Hill Pages

1-8 and 39-80

[2] Prof Khalid Mohd Nor (2006) Lecture Notes ndash MEP 1542 Special Topic

In Power Engineering session 20052006-II

[3] Tenaga National Berhad (1996) ldquoA Guidebook on Power Quality-

Monitoring Analysis amp Mitigationsrdquo pages 1-61

[4] IEEE Standards Board (1995) ldquoIEEE Std 1159-1995rdquo IEEE

Recommended Practice for Monitoring Electric Power Qualityrdquo IEEE Inc New

York

[5] IEEE Industry Applications Magazine ldquoBefore and During Voltage

sagsrdquo available at httpwwwieeeorgias

[6] ldquoSEMI F47-0200 voltage sag immunity curverdquo available at

httpwwwsemiorg

[7] ldquoITI (CBEMA) curve application noterdquo Available at

httpwwwiticorgtechnicaliticurvpdf

79

[8] M H Haque (2001) Compensation of Distribution System Voltage Sag

by DVR and D-STATCOM IEEE Porto Power Tech Conference 2001

[9] M A Hannan and A Mohamed (2002) ldquoModeling and Analysis of a 24-

Pulse Dynamic Voltage Restorer in a Distribution Systemrdquo Student Conference

on Research and Development PROCEEDINGS Shah Alam Malaysia

[10] A Hernandez K E Chong G Gallegos and E Acha ldquoThe

implementatio of a solid state voltage source in PSCADEMTDCrdquo IEEE Power

Eng Rev pp 61-62 Dec 1998

[11] L Xu Anaya-Lara V G Agelidis and E Acha ldquoDevelopment of

custom power devices for power quality enhancementrdquo in Proc 9th ICHQP

2000 Orlando FL Oct 2000 pp 775-783

[12] Y Chen and B T Ooi ldquoSTATCOM based on multimodules of

multilevel converters under multiple regulation feedback controlrdquo IEEE Trans

Power Electron vol 14 pp 959-965 Sept 1999

[13] E Acha V G Agelidis O Anaya-Lara and T J E Miller lsquoElectronic

Control in Electrical Power Systemsrdquo London UK Butterworth-Heinemann

2001

[14] K Chan A Kara and G Kieboom ldquoPower quality improvement with

solid state transfer switchesrdquo in Proc 8th ICHQP 1998 Athens Greece Oct

1998 pp 210-215

[15] PSCAD Electromagnetic Transients Userrsquos Guide The Professionalrsquos

Tool for Power System Simulation

80

[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power

systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17

(1) pp 266-272 2002

[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of

conventional and advanced static var compensators in electromagnetic transients

simulation programrdquo Available at httpwwweeumanitobaca~hvdc

[18] N Mohan T M Underland and W P Robbins ldquoPower electronics

Converters Application and Designrdquo New York Wiley 1995

80

[16] O Anaya-Lara E Acha ldquoModelling and analysis of custom power

systems by PSCADEMTDCrdquo IEEE Trans Power Delivery Vol PWDR-17

(1) pp 266-272 2002

[17] I T Fernando W T Kwasnicki and A M Gole ldquoModeling of

conventional and advanced static var compensators in electromagnetic transients

simulation programrdquo Available at httpwwweeumanitobaca~hvdc

[18] N Mohan T M Underland and W P Robbins ldquoPower electronics

Converters Application and Designrdquo New York Wiley 1995

81

APPENDIX A

Data generated by PSCADEMTDC for DSTATCOM

======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_6 4 00 NT_7 5 00 NT_8 6 00 NT_12 7 00 NT_13 8 00 NT_14 9 00 NT_15 10 00 NT_16 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 18 00 NT_24 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 1 2 RE 00 1 NT_1 NT_2 6 9 RS 10000000 1 NT_12 NT_15 6 1 RS 10000000 1 NT_12 NT_1 1 6 RS 10000000 1 NT_1 NT_12 2 6 RS 10000000 1 NT_2 NT_12 6 2 RS 10000000 1 NT_12 NT_2 7 1 RS 10000000 1 NT_13 NT_1 1 7 RS 10000000 1 NT_1 NT_13 2 7 RS 10000000 1 NT_2 NT_13 7 2 RS 10000000 1 NT_13 NT_2 8 1 RS 10000000 1 NT_14 NT_1 1 8 RS 10000000 1 NT_1 NT_14 2 8 RS 10000000 1 NT_2 NT_14 8 2 RS 10000000 1 NT_14 NT_2 7 10 RS 10000000 1 NT_13 NT_16 0 12 RE 00 1 GND NT_18 0 13 RE 00 1 GND NT_19 0 14 RE 00 1 GND NT_20 8 11 RS 10000000 1 NT_14 NT_17 16 18 RS 10000000 1 NT_22 NT_24 15 18 RS 10000000 1 NT_21 NT_24 17 18 RS 10000000 1 NT_23 NT_24 16 17 RS 10000000 1 NT_22 NT_23 17 15 RS 10000000 1 NT_23 NT_21 15 16 RS 10000000 1 NT_21 NT_22 17 0 RL 121 01926 1 NT_23 GND 15 0 RL 121 01926 1 NT_21 GND 16 0 RL 121 01926 1 NT_22 GND

82

14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE

83

APPENDIX B

Data generated by PSCADEMTDC for DVR

======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND

84

--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE

85

APPENDIX C

Data generated by PSCADEMTDC for SSTS

======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0

86

DATADSD DATADSO ENDPAGE

82

14 5 RL 01 0758 1 NT_20 NT_8 13 4 RL 01 0758 1 NT_19 NT_7 12 3 RL 01 0758 1 NT_18 NT_6 1 2 C 7500 1 NT_1 NT_2 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 3 Winding Transformer Name T1 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV V3 110 kV Imag1 002 pu Imag2 002 pu Imag3 002 pu Xl 01 01 01 (pu) Sat 0 -3 Number of windings 3 0 791831796746 11 0 -827824151144 34618100866 17 0 -827824151144 -17309050433 34618100866 888 4 0 10 0 15 0 888 5 0 9 0 16 0 DATADSD DATADSO ENDPAGE

83

APPENDIX B

Data generated by PSCADEMTDC for DVR

======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND

84

--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE

85

APPENDIX C

Data generated by PSCADEMTDC for SSTS

======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0

86

DATADSD DATADSO ENDPAGE

83

APPENDIX B

Data generated by PSCADEMTDC for DVR

======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_4 5 00 NT_5 6 00 NT_6 7 00 NT_7 8 00 NT_10 9 00 NT_11 10 00 NT_13 11 00 NT_17 12 00 NT_18 13 00 NT_19 14 00 NT_20 15 00 NT_21 16 00 NT_22 17 00 NT_23 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 5 1 RS 10000000 1 NT_5 NT_1 5 3 RS 10000000 1 NT_5 NT_3 2 0 RS 10000000 1 NT_2 GND 3 0 RS 10000000 1 NT_3 GND 1 0 RS 10000000 1 NT_1 GND 5 2 RS 10000000 1 NT_5 NT_2 5 0 RS 10 1 NT_5 GND 0 17 RE 00 1 GND NT_23 0 16 RE 00 1 GND NT_22 3 5 RS 10000000 1 NT_3 NT_5 2 5 RS 10000000 1 NT_2 NT_5 1 5 RS 10000000 1 NT_1 NT_5 0 3 RS 10000000 1 GND NT_3 0 2 RS 10000000 1 GND NT_2 0 1 RS 10000000 1 GND NT_1 11 6 RS 10000000 1 NT_17 NT_6 6 7 RS 10000000 1 NT_6 NT_7 7 11 RS 10000000 1 NT_7 NT_17 11 0 RS 10000000 1 NT_17 GND 6 0 RS 10000000 1 NT_6 GND 7 0 RS 10000000 1 NT_7 GND 0 15 RE 00 1 GND NT_21 15 10 RL 01 0758 1 NT_21 NT_13 13 0 RL 01 01926 1 NT_19 GND 12 0 RL 01 01926 1 NT_18 GND 16 8 RL 01 0758 1 NT_22 NT_10 17 9 RL 01 0758 1 NT_23 NT_11 14 0 RL 01 01926 1 NT_20 GND

84

--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE

85

APPENDIX C

Data generated by PSCADEMTDC for SSTS

======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0

86

DATADSD DATADSO ENDPAGE

84

--------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 -2 Number of windings 10 0 59387384756 11 0 -124173622672 259635756495 888 8 0 6 0 888 9 0 7 0 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 14 11 259635756495 4 1 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 12 6 259635756495 4 2 -124173622672 59387384756 Single Phase Transformer 1000 MVA 110 kV 2300 kV -2 Number of windings 13 7 259635756495 4 3 -124173622672 59387384756 DATADSD DATADSO ENDPAGE

85

APPENDIX C

Data generated by PSCADEMTDC for SSTS

======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0

86

DATADSD DATADSO ENDPAGE

85

APPENDIX C

Data generated by PSCADEMTDC for SSTS

======================================================================= Generated by PSCAD v410 Warning The content of this file is automatically generated Do not modify as any changes made here will be lost ======================================================================= --------------------------------------- Local Node Voltages --------------------------------------- VOLTAGES 1 00 NT_1 2 00 NT_2 3 00 NT_3 4 00 NT_7 5 00 NT_8 6 00 NT_9 7 00 NT_10 8 00 NT_11 9 00 NT_12 --------------------------------------- Local Branch Data --------------------------------------- BRANCHES 0 9 RE 00 1 GND NT_12 0 8 RE 00 1 GND NT_11 0 7 RE 00 1 GND NT_10 3 2 RS 10000000 1 NT_3 NT_2 2 1 RS 10000000 1 NT_2 NT_1 1 3 RS 10000000 1 NT_1 NT_3 3 0 RS 10000000 1 NT_3 GND 2 0 RS 10000000 1 NT_2 GND 1 0 RS 10000000 1 NT_1 GND 7 3 RL 01 0758 1 NT_10 NT_3 5 0 R 200 1 NT_8 GND 4 0 R 200 1 NT_7 GND 6 0 R 200 1 NT_9 GND 8 2 RL 01 0758 1 NT_11 NT_2 9 1 RL 01 0758 1 NT_12 NT_1 --------------------------------------- Local Transformer Data --------------------------------------- TRANSFORMERS 3 Phase 2 Winding Transformer Name T32 Tmva 1000 MVA Freq 500 Hz V1 2300 kV V2 110 kV Imag1 002 pu Imag2 002 pu Xl 01 pu Sat 0 2 Number of windings 3 0 00 841929648956 6 0 00 402259344016 00 0192577481141 888 2 0 4 0 888 1 0 5 0

86

DATADSD DATADSO ENDPAGE

86

DATADSD DATADSO ENDPAGE