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Power Systems Engineering Thesis

2020

Substation Design and Modeling for

Amhara Metal Industry And Machine

Technology Development Enterprise

with IEEE Standards

KAHSAY, HAILEYESUS

http://hdl.handle.net/123456789/11697

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BAHIR DAR UNIVERSITY

BAHIR DAR INSTITUTE OF TECHNOLOGY

SCHOOL OF RESEARCH AND POSTGRADUATE STUDIES

ELECTRICAL AND COMPUTER ENGINEERING FACULTY

Substation Design and Modeling for Amhara Metal Industry And

Machine Technology Development Enterprise with IEEE Standards

MSc. THESIS

BY

HAILEYESUS KAHSAY

GRADUATES PROGRAM IN POWER SYSTEMS ENGINEERING

ADVISOR: DR. -ING. BELACHEW BANTYIRGA

BAHIR DAR, ETHIOPIA

June 2020

Substation Design and Modeling for Amhara Metal Industry Machine Technology

Development Enterprise with IEEE Standards

BY

Haileyesus Kahsay

A thesis submitted to the school of Research and Graduate Studies of Bahir Dar Institute

of Technology, BDU in partial fulfillment of the requirements for the degree of master in

the Power System Engineering in the Faculty of Electrical and Computer Engineering.

Advisor:

Dr.-Ing. BELACHEW BANTYIRGA

Bahir Dar, Ethiopia

May 27, 2020

SUBSTATION DESIGN AND MODELING FOR AMHARA METAL INDUSTRY MACHINE TECHNOLOGY DEVELOPMENT ENTERPRISE WITH IEEE STANDARDS

I

DECLARATION

I, the undersigned, declare that the thesis comprises my own work. In compliance

with internationally accepted practices, I have acknowledged and refereed all

materials used in this work. I understand that non-adherence to the principles of

academic honesty and integrity, misrepresentation/ fabrication of any

idea/data/fact/source will constitute sufficient ground for disciplinary action by the

University and can also evoke penal action from the sources which have not been

properly cited or acknowledged.

Name of the student: Haileyesus Kahsay Woldemicael

Signature:

Date of submission: ___________________

Place: Bahir Dar

This thesis has been submitted for examination with my approval as a university

advisor.

Advisor Name: Dr.-Ing. Belachew Bantyirga

Advisor’s Signature:


II

© May 27, 2020

HAILEYESUS KAHSAY WOLDEMICAEL

SUBSTATION DESIGN AND MODELING FOR AMHARA METAL INDUSTRY

MACHINE TECHNOLOGY DEVELOPMENT ENTERPRISE WITH IEEE

STANDARDS

ALL RIGHTS RESERVED


III

BAHIR DAR UNIVERSITY

BAHIR DAR INSTITUTE OF TECHNOLOGY

SCHOOL OF RESEARCH AND GRADUATE STUDIES

FACULTY OF ELECTRICAL AND COMPUTER ENGINEERING


IV

ACKNOWLEDGMENTS

First of all, glory and thanks to the almighty God for the completion of academic carrier

and this master’s thesis.

Then I would like to express my deepest and sincere gratitude to my supervisor and

advisor, Dr.-Ing Belachew Bantyirga, for his stimulating guidance, constructive

comments, support and encouragement throughout the thesis work. I am really thankful

to him.

My colleague Addisu Mullat, Desalegn Abebe and my classmate have been a persistent

source of encouragement throughout my thesis work.

My special thanks to my brother Bereket Kahsay, and my classmate Asmamaw Admas, I

want them to know that their help in my carrier always kept in my memory


V

ABSTRACT

Substations are part of a power system in which voltage transformation and switching

take place. The need for building a new substation may arise to satisfy the load growth of

customers or when the number of outgoing bays that can be connected to the substation is

fully engaged. During such conditions, establishing a new substation will be a must. This

thesis is developed by observing the problem happened in Amhara Metal Industry and

Machine Technology Development Enterprise (AMIMTDE). AMIMTDE power demand

for its foundries and factories that going to be established in Bahir Dar city has been out

of the capacity of the nearby substation; Bahir Dar substation II. Consequently, the

enterprise has been forced to establish its own substation. The first task of the study is

collecting primary and secondary data about Bahir Dar substation from Ethiopian Electric

Agency and Ethiopian Electric Utility Bahir Dar District. To forecast load, data will be

also collected from AMIMTDE. The research work will include study different methods

of connections the new designed substation to the existing substation, load forecasting,

design using IEEE standards and guidelines on major substation equipment and

protection systems. Then by using ETAP software, the output of the design will be

modeled, analyze stability problems and take corrective actions when necessary. The

researcher will also use SolidWork and Auto Cad software to model lightning protection

system and to draw single line diagram of the substation. The output of this thesis may be

used as reference material by the enterprise during bidding process and for students who

need to work on power quality, transient analysis and other studies on the new substation

and on the factories.


VI

LIST OF ABBREVIATIONS AND SYMBOLS

A. Abbreviations

AMIMTDE Amhara Metal Industry and Machine Technology Development

Enterprise

ANSI American National Standard Institute

ACSR Aluminum Conductor Steel Reinforced

BB Busbar

BCT Bushing Type Current Transformer

BIL Basic Insulation Level

CAIDI Customer Average Interruption Duration Index

CAIFI Customer Average Interruption Frequency Index

CB Circuit Breaker

CVT Capacitor Voltage Transformer

CT Current Transformer

DETC De-energize Tap Changer

DigSILENT Digital Simulation of Electrical Networks

EEU Ethiopian Electric Utility

EEP Ethiopian Electric Power

EEA Ethiopian Electric Agency

EMI Electromagnetic Interference

ENS Energy Not Supplied Index

ETAP Engineering Technical Acquisition Project

FRE Frequency of Interruption

G.C. Gregorian Calendar

GOOSE Generic Object-Oriented Substation Event

HI High Voltage

IACS International Annealed Copper Standard

IEC International Electrotechnical Commission

IED Intelligent Electronic Devices

IEEE Institute of Electrical and Electronics Engineering


VII

IP Ingress Protection

LILO Line In Line Out

LN Logical Node

LTC Load Tap Changer

MAIFI Momentary Average Interruption Frequency Index

MP Microprocessor

OD Outside Diameter

OFAF Oil Forced Air Forced

ONAN Oil Natural Air Natural

ONAF Oil Natural Air Forced

NEC National Electric Code

PSS Power System Stabilizer

PT Potential Transformer

RMS Room Mean Square

SAIDI System Average Interruption Duration Index

SAIFI System Average Interruption Frequency Index

SAS Substation Automation System

SCADA Supervisory Control And Data Acquisition

SPS Standard Pipe Size

SS Substation

TCP Transmission Control Protocol

TCF Transformer Correction Factor

B. Symbols

3∅ Three phase

f Frequency

I Current

P Real Power

Q Reactive Power

V Voltage


VIII

TABLE OF CONTENTS

DECLARATION ................................................................................................................................... I

ACKNOWLEDGMENTS ..................................................................................................................... IV

ABSTRACT ......................................................................................................................................... V

LIST OF ABBREVIATIONS AND SYMBOLS ......................................................................................... VI

A. Abbreviations ...................................................................................................................... VI

B. Symbols .............................................................................................................................. VII

LIST OF FIGURES .............................................................................................................................. XI

LIST OF TABLES ............................................................................................................................... XII

CHAPTER ONE .................................................................................................................................. 1

Introduction ..................................................................................................................................... 1

1.1. Background ...................................................................................................................... 1

1.1.1 Background information about AMIMTDE ............................................................ 4

1.2. Literature Review ............................................................................................................. 5

1.3. Statement of the problem ................................................................................................. 7

1.4. Objectives of the study .................................................................................................... 8

1.4.1 General Objective .................................................................................................... 8

1.4.2 Specific Objective ..................................................................................................... 8

1.5. Methodology .................................................................................................................... 8

1.6. Scope ................................................................................................................................ 9

1.7. Significance of the study .................................................................................................. 9

1.8. Organization of the thesis ................................................................................................ 9

CHAPTER TWO ............................................................................................................................... 11

2. LOAD FORECASTING AND OPTIONS FOR CONNECTING AMIMTDE SUBSTATION WITH BAHIR

DAR SUBSTATION II ........................................................................................................................ 11

2.1. Introduction ................................................................................................................... 11

2.2. Load forecasting ............................................................................................................. 12

2.3. Connection of the AMIMTDE Substation with Bahir Dar substation II .......................... 16

2.3.1 Options to connect AMIMTDE substation with Bahir Dar substation II ................ 18


IX

CHAPTER THREE ............................................................................................................................. 20

3. SUBSTATION DESIGN AND SPECIFICATIONS .......................................................................... 20

3.1 Introduction .................................................................................................................... 20

3.2 Busbar arrangement ....................................................................................................... 20

3.2.1 Single busbar system ............................................................................................. 21

3.2.2 Single busbar system with section ......................................................................... 22

3.2.3 Main and transfer bus arrangement ...................................................................... 23

3.2.4 Double busbar system ............................................................................................ 24

3.2.5 Ring bus .................................................................................................................. 25

3.2.6 Breaker and half system ......................................................................................... 26

3.3 Busbar arrangement selection ........................................................................................ 29

3.4 Selection of busbar construction type ............................................................................ 29

3.5 Selection of busbar type ................................................................................................. 30

3.6 Busbar sizing .................................................................................................................. 31

3.6.1 Convectional heat loss ........................................................................................... 34

3.6.2 Radiation heat loss ................................................................................................. 34

3.6.3 Solar heat gain ........................................................................................................ 35

3.6.4 Voltage gradient ..................................................................................................... 38

3.7 Selection of transformer ................................................................................................ 43

3.7.1 Voltage Drop at Transformer ................................................................................. 45

3.8 Transmission line ........................................................................................................... 46

3.9 Selection of Isolators ...................................................................................................... 47

3.9.1 Allowable continuous current of isolators ............................................................. 47

3.9.2 Rated short-time withstand current ........................................................................ 49

3.10 Selection of Circuit Breakers .......................................................................................... 50

3.11 Selection of Instrument Transformer ............................................................................ 51

3.11.1 Current Transformer .............................................................................................. 53

3.11.2 Potential Transformer ............................................................................................ 54

3.12 Surge Arrestor Selection ................................................................................................ 55

3.13 Lightning stroke shielding .............................................................................................. 56

3.14 Earth Mat Design ........................................................................................................... 61


X

3.14.1 The resistance of human body ............................................................................... 62

3.14.2 Data collection on soil resistivity ........................................................................... 62

3.14.3 Effect of surface material ....................................................................................... 63

3.14.4 Calculation of maximum grid current .................................................................... 64

3.14.5 Diameter of grid conductor .................................................................................... 67

3.14.6 Grid resistance calculation ..................................................................................... 69

3.14.7 Maximum mesh voltage ......................................................................................... 72

3.14.8 Step voltage ............................................................................................................ 75

3.15 AMIMTDE Substation Layout ...................................................................................... 78

CHAPTER FOUR .............................................................................................................................. 80

4. SIMULATION RESULT AND DISCUSSION ................................................................................ 80

4.1 Introduction .................................................................................................................... 80

4.2 Load flow analysis .......................................................................................................... 83

4.2.1 Load flow analysis of Bahir Dar Substation II ............................................................. 83

4.2.2 Load flow analysis of Bahir Dar SS by connecting AMIMTDE SS ................................ 84

CHAPTER FIVE ................................................................................................................................ 89

5. CONCLUSION AND RECOMMENDATIONS .............................................................................. 89

5.1 Conclusion ..................................................................................................................... 89

5.2 Recommendation ........................................................................................................... 90

5.3 Future Work ................................................................................................................... 90

BIBLIOGRAPHY ............................................................................................................................... 91

Appendix ........................................................................................................................................ 96


XI

LIST OF FIGURES

Figure 1: Components of substation ................................................................................... 1

Figure 2: Different criteria for substation design ................................................................ 2

Figure 3: Automated substation system .............................................................................. 3

Figure 4: Site plan of AMIMTDE .................................................................................... 15

Figure 5: Single line diagram of Bahir Dar Substation II ................................................. 17

Figure 6: Single busbar with incoming and outgoing bays............................................... 22

Figure 7: Single busbar with sectionalization ................................................................... 23

Figure 8: Main and transfer busbar arrangement .............................................................. 23

Figure 9: Double busbar with single breaker system ........................................................ 24

Figure 10: Double busbar with double breaker system .................................................... 25

Figure 11: Ring bus system............................................................................................... 26

Figure 12: Breaker and half arrangement with two main busbar...................................... 27

Figure 13: Protection of substation using mast with fixed angle method ......................... 58

Figure 14: Empirical Curve with 0.1 and 1 percent exposure .......................................... 58

Figure 15: Lightning protection using rolling sphere ....................................................... 60

Figure 16:Substation protected from lightning ................................................................. 61

Figure 17: Collecting data on site ..................................................................................... 63

Figure 18: Input Data to ETAP software .......................................................................... 70

Figure 19: Layout of ground grids and rods ..................................................................... 70

Figure 20: Single line diagram of Bahir Dar II and AMIMTDE substations. .................. 82

Figure 21: Load flow simulation of Bahir Dar SS II without AMIMTDE substation. ..... 83

Figure 22: Load flow simulation of Bahir Dar SS II with AMIMTDE SS....................... 85


XII

LIST OF TABLES

Table 1 Current power rating of AMMITDE ................................................................... 12

Table 2 Forecasted power demand of AMIMTDE ........................................................... 14

Table 3 Comparison of major substation equipment ........................................................ 19

Table 4 Advantage and disadvantage of different bus arrangements ............................... 27

Table 5 Values of emissivity and absorptivity for copper and aluminum ........................ 35

Table 6 Specification for transformers ............................................................................. 45

Table 7 Temperature limit for Isolators ............................................................................ 48

Table 8 Specification of Isolator ....................................................................................... 49

Table 9 Specification for Circuit Breaker ......................................................................... 50

Table 10 Dielectric strength correction factors for altitudes greater than 1000m ............ 51

Table 11 The standard accuracy classes ........................................................................... 53

Table 12 Specification for current transformer ................................................................. 53

Table 13 Specification for potential transformer .............................................................. 54

Table 14 Ratings for surge arrester ................................................................................... 56

Table 15 Basic Data to calculate stroke distance .............................................................. 60

Table 16 Range of earth resistivity ................................................................................... 62

Table 17 Typical values of decrement factor referred from IEEE std 80-2013................ 66

Table 18Ground conductor sizing factor .......................................................................... 68

Table 19 Load flow on Bahir Dar SS by disconnecting AMIMTDE SS .......................... 84

Table 20 Load flow analysis summary on busbars operating voltageby disconnecting

AMIMTDE SS .................................................................................................................. 84

Table 21 Loads connected to AMIMTDE Substation ...................................................... 84

Table 22 Power and PF on Bahir Dar SS II when AMIMTDE SS connected.................. 86

Table 23 Load flow analysis summary on busbars operating voltage with AMIMTDE SS

connected. ......................................................................................................................... 86

Table 24 Difference in Operating voltage and %PF ......................................................... 87


1

CHAPTER ONE

INTRODUCTION

1.1. Background

A substation is a station in the power system at which electric power is transformed into a

conveniently used form. The station consists of transformers, switches, circuit breakers

and other auxiliary equipment. The general functions of substations include:

voltage transformation

connection point for transmission lines

switchyard for network configuration

monitoring point for control center

protection of power lines and apparatus

Communication with other substations and regional control centers.

All substation equipment can be divided into three groups: major, auxiliary and control

system equipment. Major equipment includes power transformer, switching equipment,

instrument transformers, overvoltage protection equipment as shown in Figure 1 [1].

Control equipment includes relay protection systems, metering systems, alarm and

remote control systems. Auxiliary systems equipment are typically used to supply loads

such as transformer cooling, oil pumps, and load tap changers, circuit breaker air

compressors and charging motors, outdoor device heaters, outdoor lightings and motor-

operated disconnecting switches [2].

A. Busbar

B. Isolator

C. Circuit Breaker

D. Current Transformer

E. Voltage Transformer

F. Earthing Switch

G. Surge Arrestor

Figure 1: Components of substation


2

Substation shall be designed economically to minimize the investment cost but with a

reliable service to the customer, safety to personnel and equipment. Reliability is defined

by the International Electrotechnical Commission as the ability to perform a required

function under given conditions for a given time interval. The reliability of the power

system can be improved by either shortening the duration of interruptions of the power

supply or decreasing the frequency by which interruptions occur [3].

Depending on the size and functions performed by a substation, the configuration and

ratings of equipment can be varied. The design needs to consider the present and future

needs that the substation will provide. By selecting the appropriate design, configuration

and calculate the ratings of main equipment such as busbars, transformers and switchgear

will ensure to serve the expected service years with no or less trouble-free service [4].

To design a substation with better technical performance and reduced cost, one needs to

meet the following criteria: reliability, security, interoperability, re-configurability,

controllability, maintainability, flexibility, reduced cost and environmental impact. An

estimate of the importance of the different criteria is shown in figure 2 with pie chart [1].

Figure 2: Requirements for substation design

Re-configurability

Security

Reliability

Interoperabil

ity

Controllability

Flexibility

Maintainability

Cost

Environmental impact


3

The reliability factor considers the main factor during new substation design but it should

be compared with cost. The reliability of a substation can be defined as the ability of a

power system to fulfill its function [5]. Reliability can be studied by the failure rate of the

system and the downtime during maintenance. The reliability of a power system is

mainly dependent on three factors: design, installation and the deterioration in service [6].

The security of a substation can be seen in two ways: The first one is its ability to afford

disturbance which is related to transient behavior and the other is the security related to

the automation system.

Substation Automation System (SAS) deals with the act of automatically controlling the

power system via instrumentation and control devices. It acts by using data from IEDs

and control power system devices from remote [7].

Figure 3: Automated substation system

One of the standards which widely used about substation automation is IEC 61850. It

deals with interoperability between IEDs for best protection, monitoring and control of


4

substation systems. There are also IEEE standards for substations standards which

included active standards covering switching stations, transformer stations, and

generating stations. IEEE Substations Standards are a single source for design

construction and operation of power substations. IEEE substations standards contain

around 50 active IEEE standards, guides and recommended practices for power

substations [8]

1.1.1 Background information about AMIMTDE

Amhara Metal Industry and Machine Technology Development Enterprise (AMIMTDE)

is established in Bahir Dar city in 2016 G.C. with the primary aim of enhancing the

transition of the region from agricultural to industrial leading economy by producing

capital goods and establishing different factories in the region with turnkey projects.

Currently, the enterprise is establishing ferrous and nonferrous foundries and general

workshop which consists of different shops in Bahir Dar town around kebele 14

industrial zone site.

The ferrous and nonferrous foundries each consists of two melting sections that use

inductive melting methods. The general workshop and farm implement will have a

machine shop, sheet metal processing shop, forging and heat treatment shop, welding

shop, wood workshop and assembling shop [9].

The electric power to run the above-mentioned workshops and foundries was supposed to

be obtained from the nearby substation, Bahir Dar substation II which is found around 11

km away but its power need is beyond the capacity of 66/33kV,12.5 MVA transformer of

the substation. Therefore, AMIMTDE forced to design and construct its own substation

for its own load and for other industries that are going to be constructed around it.

Bahir Dar substation II is connected from Tis Abay and Tana Beles hydropower

generation stations with 132 and 400 kV transmission lines. The new substation will have

several possible connection systems like tapping, LILO or interconnected system. By

considering the appropriate techniques and cost the researcher will choose the type of

connection that the new substation will have and continue the design and modeling of the

substation after going through IEEE and other international standards.


5

The design part will include calculating the ratings of substation equipment like busbars,

isolators, circuit breakers, current transformers, voltage transformers, surge arrestors,

grounding and lightning protection. For modeling, the researcher will use ETAP

software.

1.2. Literature Review

A lot of researches have done in the area of substations like smart substation, mobile

substation design, improving the reliability of substation and automation systems.

Particularly research papers that focused on substation design, protection systems and

reliability are discussed as follows.

2018, Tomohiro Oowaku. The paper describes the experiment conducted on the

response of grounding system for surge current and voltage with the aim of increasing the

tolerance of traction substation against lightning. The experiment is conducted around

Tokyo metropolitan area. The experiment focuses on the influence of the grounding rods

position, surge injection point on the overhead grounding wire environmental and

influence of overhead grounding wire configuration. The experiment conclude that the

grounding point of the overhead grounding wire to which the surge current is applied

influences on the transient overvoltage value strongly and by modifying overhead

grounding wire configuration it will be possible to guide surge current to the grounding

point with mesh far from the electric machine which should be protected from the

lighting and to suppress the transient overvoltage at the machine [10].

2017, Semen Lukianov. The paper is master thesis that presents about design of power

substation. The researcher presents a methodology that can be used for different network

development alternatives based on the technical and economical point of view which

includes the lifetime cost analysis. The researcher presents different types of power

substation, their functions and different busbar structures. Besides, the features and steps

of power substation planning are discussed finally the researcher applied the developed

methodology to a region that selected for the case study but the author only do the

necessary calculation and methodology didn't put the new substation system in single line

diagrams and didn't check the design with simulation software [11].


6

2017, Balduino E., Mugilila C., Oana C., Loretta I. The authors discuss one of the

standards of IEC: IEC 61850 which explains the standard for substation automation. The

author considers the functionality and requirements of the standard. The authors develop

an application that is planned to check how the IEC protocols interface with Intelligent

Electronic Devices (IEDs). The work highlights the importance of modern IEDs,

emphasizing the main characteristics that these components must possess to meet the

requirements of IEC 61850 through the implementation of Logical Nodes (LNs) and

generic Object Oriented Substation Event (GOOSE)messages. During their practical

experiments on 110/20kV Babadag1 wind farm, the authors found that IEC 61850

protocol which used as a Server/ Client configuration showed better transmission times

and behavior better suited for critical applications than Mod bus protocol for substation

automation but the authors didn't compare with economic analysis IEC 61850 and older

standards [12].

2017, Jun Wang. The author presents a new architecture of protection in substation

which consists of unit protection, substation area protection and wide area protection. The

author proposes an algorism that uses the traditional differential current principle and can

accurately locate the fault. The author uses digital signal processor and designed the

architecture which is in line with IEC 61850 [13].

2015, Bruce Pickeh. The paper addresses the subject of reducing outages or duration of

outages by using incipient detection methods that alarm or de-energize a piece of

equipment prior to the all substation failure. The researcher listed the proactive methods

which prevent the event from occurring or predictive action based upon trending

streaming data and set points and reactive methods that take control action by providing

real-time information. The methods and technique have been mentioned for major

substation equipment with most of the control systems mentioned is based on a

multifunctional MP relay system. Detection includes noise in connection, partial

discharges in transformers that are arcs within the voids, phase angle and phase

magnitude change detection for CVTs [14].


7

2014, Behailu Abebe. The case study paper focuses on confirming the reliability

problem of Bishoftu distribution substation II and solves the problem by designing a new

distribution substation. The researcher confirmed the existence of the reliability problem

by collecting the frequency of planned and none momentary interruptions data from the

substation and analyze with reliability indices of SAIFI, SAIDI, MAIFI and ENS value

then compare the result with EEPCO and other countries. The result shows Bishoftu

Substation II has serious reliability and has also overloading problems. To solve this, the

researcher designs a new substation. But the design didn't follow IEEE design procedures

like the design of the busbar didn't consider the operating temperature effect with the

current carrying capacity [15].

2011, Reza Vafamehr. The paper presented is a case study of a master thesis about the

design of the electrical power supply system for an oil and gas refinery. The author

discusses the load flow analysis, dimensioning of transformers, conductor size,

verification of voltage drop limits, short circuit calculations, selection of circuit breakers,

and verification of the protection of conductors. But the author doesn't specify particular

methods or what special attention should be given to select equipment because the

refinery is a special hazardous area [16].

1.3. Statement of the Problem

Ethiopia is gifted with abundant renewable energy resources and has the potential to

generate over 45,000MW of electric power from hydroelectric, solar, geothermal

and wind sources and currently, we have 2,300MW of installed generation capacity [17].

Following this, the federal government and regional governments are establishing

industrial parks in different regions that need new transmission systems and substations.

Likewise, AMIMTDE which established by Amhara Regional Government needs a new

substation for its foundries and factories because the power required by the enterprise,

50MVA, is beyond the capacity of the nearby substation Bahir Dar substation II 33kV

busbar which is 12.5MVA.


8

The focus of the thesis is to design and model a substation for the above mentioned

enterprise by using IEEE standards, guide lines and recommended practices. IEEE has

more than 50 standards and guide lines which focus on building reliable substation

economically.

The output of this thesis will create awareness about substation, major substation

equipment and the necessary protection systems for management staffs of the enterprise.

The thesis may also be used by the enterprise for material selection in the bidding

process, for supervision during installation and as reference material for other students

who need to work on stability, transient analysis and other studies on the new substation.

1.4. Objectives of the Study

1.4.1 General objective

Design and modeling of substation for AMIMTDE by using IEEE standards.

1.4.2 Specific objective

To study different techniques to connect AMIMTDE substations to Bahir Dar

Substation II.

To study different types of load forecasting systems.

To study parameters used in design substation equipment, grounding and

protection system.

To model the substation system using ETAB software.

Design grounding systems

Design lightning protection system

1.5. Methodology

To design AMIMTDE substation the researcher will follow the following procedures.

Literature Review


9

The researcher will revise different journals, articles, thesis, books and manuals

about design and modeling of substation systems including IEEE and other

important standards that explain how substation designs.

Data Collection

The researcher will collect primary and secondary data from AMIMTDE to know

the total KVA load of the enterprise and how much voltage that needs especially

the foundries since some induction type foundries need high voltage like 33kV.

Design and modeling

Estimate the current and future load of the enterprise.

Specify the necessary equipment for the substation and calculate the rating

of the equipment.

Modeling and check the designs using ETAP software.

1.6. Scope

The focus of the paper is to design and model a new substation for AMIMTDE. To

design efficient substation the researcher will use IEEE and other international standards.

1.7. Significance of the Study

The first beneficiary of the study is the enterprise, AMIMTDE and since the Federal and

regional governments are establishing industrial parks in the Amhara region, each park

based on the nearby substation capacity may need establishing their won substation. The

design methods of the study may be used as a guide for establishing other substations in

the region. In addition to this, the study will be input for other students who want to work

on the new substation in voltage stability, transient stability and other kinds of studies.

1.8. Organization of the Thesis

This thesis is organized in six chapters. Chapter one presents background about

substation, the enterprise AMIMTDE and gives information why they need new

substation. Chapter one will also include literature review, problem statement, objectives,


10

methodology, scope and significance of the study. The next chapter, chapter two present

load estimation of the enterprise and the connection options of the new substation with

the existing substation. Chapter three presents overview of different busbar

configurations, their advantage and disadvantage. Chapter four is about design of the

substation by using IEEE standards and guidelines, it presents the major substation

equipment sizing and protection form lightning and earth mat design. Chapter five

presents the effect of the new substation to the existing one. It deals about steady state

stability and the correction of the occurred lagging power factor by adding capacitor.

Chapter six is the last chapter which has conclusions, recommendation, suggestions for

future work and Appendix. List of references are also provided at the end.


11

CHAPTER TWO

2. LOAD FORECASTING AND OPTIONS FOR CONNECTING

AMIMTDE SUBSTATION WITH BAHIR DAR SUBSTATION II

2.1. INTRODUCTION

The forecasting of load is an essential and primary task in power system development and

a basis for substation design. The estimation of the load should be exact as possible to be

cost-effective and to make transformers to work on their efficient point. There are

different methods of load forecasting among them the majors are listed below [18].

Load forecast with a load increase factor

Load forecast with economic characteristics

Load forecast with an estimated value

Load forecast with specific loads and degree of electrification

Load forecast with standardized load curve

Load forecasting using artificial intelligence algorithms such as neural networks,

fuzzy logic.

To forecast the future power demand with load increase factor we need to study the past

years' power growth and estimate the future by analyzing the growth trend. Load forecast

with economic characteristics needs to select appropriate economic variables that will be

used input to study the future demand statistically. Load forecast using estimated value

uses actual future load estimated based on internal planning obtained from large power

consumers like factories. The data can be get from land development plan, planning

authorities.

Load forecast with standardized load curves method is forecasting the future power

demand by preparing load consumption profiles of customers group which are grouped

based on their power consumption amount then by calculating the new expected

customers from each group it will be possible to forecast future demand.


12

2.2. Load Forecasting

To determine the capacity of the AMMITDE's substation it is needed to know the current

load and the future power demand or the demand because of the growth of the factories.

According to the data obtained from the enterprise electrical design case team, the current

power ratings of the machines are listed in the table below.

Table 1 Current power rating of AMMITDE

S/N Description KVA Remark

1 Nonferrous furnace 1 2000

2 Nonferrous furnace 2 2000

3 Ferrous furnace 1 2250

4 Ferrous furnace 2 2250

5 Pattern core and mold making on non-ferrous 230.4

6 Shot blasting, heat treatment and cleaning section

for non-ferrous 500.4

7 Crane, pouring system and preheater section for

non-ferrous 180

8 Pattern section for non-ferrous 384.75

9 Laboratory section 315

10 Pattern, core mold making section on ferrous 230.4

11 Full automatic resign sand reclamation section for

ferrous 540

12 Crane, pouring system and preheater for ferrous 300

13 Sheet metal fabrication 600

14 Machine shop 542.7

15 Welding and gas cutting shop 1500

16 Wood workshop 185.4

17 Assembly shop 270

Total 14279.05


13

To estimate the future power demand growth of the enterprise it is necessary to see the

strategic plan of the enterprise and the available area for possible future expansion.

Currently, the enterprise has started constructing shade for foundries and general

workshops but according to its strategic plan, the future aim is to come from general to

specific workshops or to have shops which are specialized in producing one type of

machinery. This indicates that the enterprise will expand and its power demand will also

increase.

When coming to the area available around the foundry and general workshops, the

enterprise received 10.05 hectares of land from Bahir Dar municipality around kebele 14

which is cited for industry zone but it uses only 2.8 hectares and it has 7.25 hectares for

future expansion. Now the problem is to know how much its power demand will grow

for a particular certain period.

The period or the years that the power demand will be forecasted shall be determined by

considering the life span of main equipment of the substation. Among all the major one

of the equipment are transformers. There for the life span of the substation is considered

by the life span of the transformer. Here the life of power transformer mostly depends or

affected by external factors like temperature but according to IEEE guide it assumes the

normal life of power transformer as 180000 hours which is 20 years. Therefore, the

researcher uses the forecast period for 20 years [19].

Now the problem arises by how much the enterprise power demand will grow in the next

twenty years? To know this the researcher has used the growth estimation of the

enterprise strategic plan which is 6%. Fortunately the World Bank study shows that sub

Saharan countries energy growth is 6.3% per annum [15].

Based on the above information the researcher assumes the enterprise power demand will

grow 6 percent each year. Therefore the energy demand growth for 20 years listed below

on the table.


14

Table 2 Forecasted power demand of AMIMTDE

S/No Years

power demand

of the last year

(MVA)

Growth rate

(MVA)

Estimated

power demand

of the year

(MVA)

1 2019 14.28 0.86 15.14

2 2020 15.14 0.91 16.04

3 2021 16.04 0.96 17.01

4 2022 17.01 1.02 18.03

5 2023 18.03 1.08 19.11

6 2024 19.11 1.15 20.26

7 2025 20.26 1.22 21.47

8 2026 21.47 1.29 22.76

9 2027 22.76 1.37 24.12

10 2028 24.12 1.45 25.57

11 2029 25.57 1.53 27.11

12 2030 27.11 1.63 28.73

13 2031 28.73 1.72 30.46

14 2032 30.46 1.83 32.28

15 2033 32.28 1.94 34.22

16 2034 34.22 2.05 36.27

17 2035 36.27 2.18 38.45

18 2036 38.45 2.31 40.76

19 2037 40.76 2.45 43.20

20 2038 43.20 2.59 45.79

As the estimation shows in Table 2, the load on the new substation is 45.79 MVA but the

standard transformer near to 45.79 is 50MVA [20].


15

Figure 4: Site plan of AMIMTDE


16

2.3. Connection of the AMIMTDE Substation with Bahir Dar Substation II

Before going to rating and design of the substation, the researcher should go through the

arrangement of existing Bahir Dar substation II and study how the new substation will be

connected.

Bahir Dar substation II is connected with Tana Beles and Tis-Abay generation stations.

The generated power from Tana Beles is transmitted to Bahir Dar substation II with 400

kV double circuits and from Tis-Abay with 132 kV single circuit.

Generally, Bahir Dar substation II has double busbars of 400, 230 and 132 kV and single

busbar of 66 and 33 kV busbar systems. In the 400 kV double busbar system, two

incoming bays from Tana Beles generation station and one outgoing bays to Debre

Markos substation is connected.

In 230 kV double busbar two incoming bays connected from 400/230 kV transformers.

230kV busbar has four outgoing feeders that go to Motta, Alamata, Gondar I and II

substations. The 230kV double busbar connected with 132kV and 66kV busbars through

230/132/15 and 230/66/15 kV transformers respectively.

The 66 kV single busbar has one incoming circuit from 230 kV busbar through 230/66/15

kV transformer and It has three outgoing bays to Dangla, Bahir Dar I substation and the

last one connected with 33 kV single busbar.

The 33 kV busbar is connected with 66 kV busbar through 12 MVA transformers and it

has four outgoing bays. One of the busbar is connected to Gonj Kolela and the other three

are free.


17

Debremarkos Beles I Beles II

Mo

tta

Ala

ma

ta

Go

nd

ar

I

Go

nd

ar

II

Sh

un

t re

acto

r

40

0K

V

45

Mva

r

T1

40

0/2

30

KV

T2

40

0/2

30

KV

23

0/6

6/1

5K

V

63/4

0/2

3M

VA

23

0/1

32

/15K

V

63/2

1/2

1M

VA

23

0/1

32

/15

KV

63

/21

/21M

VA

400KV

Busbar

230KV

Busbar

Dangla Bahir Dar I

Gonj

Kolela

66KV Busbar

33KV Busbar

66/33KV

9.6/12MVA

132KV

Busbar

Tis Abay I 132KV Tis Abay II 132KV

Figure 5: Single line diagram of Bahir Dar Substation II


18

2.3.1 Options to connect AMIMTDE substation with Bahir Dar substation II

The first option to connect the new substation with Bahir Dar substation II is by

connecting from the busbars which have free outgoing bay and enough load-carrying

capacity. As described above the busbar which has free bay is 33 kV busbar but its

capacity is 12 MVA which is less than the load of AMIMTDE. Therefore the only

possible way of connection is from the transmission lines either that comes from Tis-

Abay (132kV line) or Tana Beles (400kV) hydropower.

Connecting with the higher voltage has the following advantages and disadvantages.

Advantage of transmitting with higher voltage.

1. Less power loss caused by I2R since in higher voltage transmission there is a

lower current.

2. The cost associated with cable size also reduces because the cable cross-section

is related to the current required to carry.

3. Voltage regulation improves since lower voltage drops due to low current.

But it has also the following disadvantages:-

1. The high cost of a step up and step down transformers and associated

protection equipment.

2. Increase the cost of line support because when increasing the voltage the

insulation required between the conductors and the earthed tower increased.

3. Additional cost due to the increase of cross arm length.

By considering the above advantages and disadvantages, the main points that the

researcher considers as decisive points to select the connection systems are transmission

losses and the cost of main substation equipment.

Comparison of losses between the connection system in 400kV and 132kV transmission

lines.

KWh loss in the life span of the substation = 2

2 2

/ 24 360 20P R Phase Km

V Pf

(1.1)


19

The active power capacity of the substation is 44.09 MW and for 400 kV line, the line

resistance is 0.0244 ohm/km and for 132 kV line 0.0269 ohm/km. By using the above

information the loss if connected to 400kV and 132kV calculated as follow;

P (loss if 400kV line selected) = 2

2 2

/ 24 360 20P R Phase Km

V Pf

(1.2)

= 2

2 2

44.09 0.0244 24 360 2063.24

400 0.9MWh

(1.3)

P (loss if 132kV line selected) = 2

2 2

44.09 0.0269 24 360 20640.24

132 0.9MWh

(1.4)

The power losses between the two options of connection system when calculated for the

expected life span (20 years) are 577 MWh. For comparison reason, it should be changed

to monetary value. To change to monetary value the researcher uses the tariff of Ethiopia

Electric Utility of the 7th

level which is applied for customers that use more than

500KWh and its value is 2.481 Birr/kW. Based on this the loss between the two options

will be 1,431,537.00 Birr.

The next step comparing with the price of major substation equipment based on the price

obtained from the Alibaba website with an exchange rate of 1 dollar equals 32.22 Birr.

Table 3. Comparison of major substation equipment

S/No Description Price if connected with

400kV line (Birr)

Price if connected with

132kV line (Birr)

1 Power transformer 45,833,000.00 23,222,000.00

2 Air circuit breaker 6,921,390.17 3,532,315.23

3 Current transformer 520,558.28 280,368.09

4 Voltage transformer 288,407.36 192,515.33

5 Isolator 115,312.76 98,898.06

Total price 53,678,668.57 27,326,096.71

Based on the above two comparison points connecting in 132kV busbar is cost effective.


20

CHAPTER THREE

3. SUBSTATION DESIGN AND SPECIFICATIONS

3.1 Introduction

Substations are equipped with large and very expensive equipment therefore it needs to

be designed to efficiently deliver reliable service with the possible lowest costs. The

design needs to follow internationally accepted standards, guidelines and best practices.

There are different internationally accepted standards like ANSI, IEC, IEEE, RUS,

ASTM, among them this thesis basis on IEEE standards and guidelines. IEEE Power and

Energy Society have prepared detail standards for each design activities. The main

standards that this thesis will follow are:-

IEEE Std 80™-2000, IEEE Guide for Safety in AC Substation Grounding

IEEE Std 81™-2012, IEEE Guide For Measuring Earth Resistivity, Ground

Impedance, And Earth Surface Potentials of a Ground System

IEEE Std 605™-2008, IEEE Guide for Design of substation Rigid-Bus Structures

IEEE Standards Interpretation for IEEE Std 605™-2008 IEEE Guide for Bus

Design in Air Insulated Substations

IEEE Std 998™-2012, IEEE Guide for Direct Lightning Stroke Shielding of

Substations

IEEE Std 1267™-1999 (R2005), IEEE Guide for Development of Specification

for Turnkey Substation Projects

3.2 Busbar Arrangement

In substations, busbars are a structure for incoming and outgoing circuit termination and

it is a means for electrically interconnecting switching and other equipment such as

transformers, capacitor banks, reactors, and so on to the system.

The selection of a busbar arrangement depends on the voltage level, the required

flexibility and cost. The arrangement selection determines the supply reliability, security


21

of the system capability to withstand short circuit, complexity of maintenance,

operational and investment cost.

According to IEEE standard (IEEE Std 605-1998) the following points area milestone for

selecting busbar arrangements.

1. Maintenance should be possible without interruption of supply.

2. The selected layout should accommodate future expansion if the load demand is

increased.

3. It needs to be economical by keeping the reliability and continuity of supply.

4. It should not provide any danger to operating personnel while doing preventive or

repairing maintenance.

5. The busbar arrangement should be simple

The most common substation busbar arrangements are mentioned and described below.

3.2.1 Single busbar system

A single busbar system consist single busbar on which all incoming and outgoing bays

are connected to it. The incoming bays are connected through isolators, circuit breakers,

instrument transformer (current and potential transformer), earth switch and outgoing

bays are connected through a transformer and they have isolators, circuit breakers,

current transformers and potential transformer. Among all systems, it is the simplest and

cheapest of all but it is not possible to maintain or make extension without shutdown of

supply. This arrangement is used for system voltages up to 33 kV. Especially indoor 11

kV substations use this arrangement.

A single busbar system can be installed with or without section. The easiest and the

chipset way of arrangement are without Section. This can be done by connecting one

incoming bay with one or more transformers.


22

Figure 6: Single busbar with incoming and outgoing bays

Depending on the incoming and outgoing lines the substation with single busbar divided

into two, terminal and through substation. In terminal substation scheme, the supplying

line terminates or ends on the substation but through substation, the incoming supply line

passes the substation at the same voltage level or the substation is tapped from the

passing line [21].

3.2.2 Single busbar system with section

In single busbar system single busbar sectioned by using a circuit breaker and isolators so

that each section act as a separate busbar. Sectionalization ensures continuity of the

supply on the other feeders during the time of maintenance or repair of one side of the

busbar. The other advantage of sectionalization is that the circuit breakers of low

breaking capacity can be used in the section as compared to without section and the bus

sectionalizer and the two incoming circuit breakers are so interlocked that only two

breakers can be operated at a time. If any of the incoming lines are between the two

sections are taken out, then the sectionalizer breaker can be closed and supply can be

retained in the section [22].


23

Figure 7: Single busbar with sectionalization

3.2.3 Main and transfer bus arrangement

In main and transfer bus arrangement two busbars one as main and the other as reserve

connected by tiebreaker and isolator. During normal operation, only main busbar is

energized and all loads are connected to it. When maintenance required on any circuit

breaker then changeover from main bus to transfer bus can be done by controlling though

bus coupler breaker. This arrangement has a disadvantage of fault in main busbar will de-

energize all the circuits there for its reliability is less and the additional isolator switch in

each bay increases its cost and land use.

Figure 8: Main and transfer busbar arrangement


24

3.2.4 Double busbar system

3.2.4.1 Double busbar system with single breaker

In this system, we have two busbars which can be called as main and spare. Each of these

two busbars can take the entire load of the system. The two busbars are connected to each

other with busbar coupler which consists of circuit breakers and isolators. The circuit

breaker provides on load change from one busbar to the other.

The advantage of this system is the reliability of the power supply is enhanced than

previously mentioned because the load can get power in any of the busbars if any one of

them is failed or when under maintenance. But the relative cost is higher than single

busbar systems and to maintain the circuit breaker on the coupler we need to interrupt the

supply. Substations with a voltage level greater than 33 kV frequently use double busbar

systems.

.

Figure 9: Double busbar with a single breaker system

3.2.4.2 Double busbar with double breaker system

Double busbar with double breaker system has two main busbars with two circuit

breakers and four switches for each incoming and outgoing bays. This arrangement

provides maximum reliability and flexibility. Any faulty circuit breakers or one of the bus


25

can be taken for maintenance without interruption to the system. But it is the most

expensive and mostly used for very large generating stations.

Figure 10: Double busbar with double breaker system

3.2.5 Ring bus

In-ring bus system the end of the busbars are returned upon themselves to form a ring

and circuit breakers are connected in serious connection and each incoming and

outgoing bays are connected between two circuit breakers that make to have alternative

power flow route. When a fault happens in any one of the circuits, it will be isolated

from the system by the two circuit breakers connected side by side and the system keep

on working as an open ring. This increases the reliability of the system but it has a

disadvantage of an even distribution of load and supply may happen when working as an

open ring. There for during design, the estimated load and the incoming bay should lay

side by side.


26

Figure 11: Ring bus system

3.2.6 Breaker and half system

In this system, we have two buses in which both of them are energized and between the

buses, we have three circuit breakers among the middle act as tie bus. The incoming and

outgoing bays are connected between the circuit breakers like for two bays we use three

circuit breakers. Each of the outgoing bays is feed from both the buses. With this

arrangement, any of the circuit breakers or one of the busbars can be opened for

maintenance without interruption of the supply. If the middle circuit breaker fails then the

breakers adjacent to the buses are tripped so interrupting both the circuits. But if a

breaker adjacent to the bus fails then the tripping of the middle breaker does not interrupt

power supply to circuit associated with a healthy breaker. Only the bay associated with

the failed breaker is interrupted.


27

Figure 12: Breaker and half arrangement with two main busbars

In the table below different busbar arrangements, advantages and disadvantages are

summarized [23].

Table 4 Advantage and disadvantage of different bus arrangements

Bus

arrangements

Advantage Disadvantage

SBSB

Low cost

Small land area

Easy to expand

Simple to operate

Simple protective

schemes

Low reliability

CB or Bus fault causes loss of entire

station

Breaker maintenance requires the

associated bay outage

MTB Easy to expand Low reliability


28

Allows for breaker

maintenance

Increased flexibility

over single bus

Bus or breaker fault causes loss of entire

station

Increased complexity over single bus

Complex protective scheme

Large land area required

DBSB

Allows for outage on

any one of the buses


over single bus

Allow for breaker

maintenance

Low reliability

CB or bus fault causes loss of entire station


Complex protective scheme

Large land are required

DBSB

Allows for outage on

any one of the buses


over single bus

May allow for

breaker maintenance

Medium reliability

Bus or breaker fault causes loss of entire

station


Medium cost

very large land area required

RB

High reliability

Flexible operation

Allows for breaker

maintenance

Bus fault doesn't

affect continuity of

operation

High cost

Breaker failure results in loss of two

circuits

large land area required

may split into two operation systems

B-1/2

Easy to expand

High reliability

Flexible operation

Allows for CB

maintenance

Bus fault does not

High cost

Extensive relaying and control schemes



29


operation

DBDB

Easy to expand

Very high reliability

Very flexible

operation

Allows for CB

maintenance

Bus fault doesn’t


operation

CB failure results in

loss of only one CB

High cost


Extensive relaying and control schemes

3.3 Busbar Arrangement Selection

Among the arrangements summarized in the above table, the one which is recommended

by IEEE standard 605TM

-2008 and used as milestone should full fill the factors of safety,

reliability, cost, flexibility of operation, ease of maintenance, available ground area,

location of connecting lines, provision for expansion, and appearance. The one which full

fill the above-mentioned criteria is DBDB scheme. It has a high advantage and fewer

disadvantages. When I consider the disadvantage of DBDB for the case of AMIMTDE,

the required land area is not as such a problem but the high cost should be considered.

The researcher's idea is to design with DBDB scheme but the enterprise can do with

single bus single breaker scheme and leave the other busbar and its coupling system as

future expansion work when they demand it.

3.4 Selection of Busbar Construction Type

The selection of construction types depends on bus configuration types, voltage level and

resources like land. There are three kinds of busbar construction types for substation

configuration: box structure, strain bus and low profile rigid bus.


30

Box structure bus is used for 110 kV and lower but it is compact or requires less amount

of land. Strain bus construction type applies for high voltage level including extra and

ultra-high tensions and it uses flexible conductors but it needs higher structures due to

conductor sag and requires more phase to phase clearance because of a swing of

conductors due to wind and short circuit forces.

Low profile rigid buses are used for all kinds of voltage levels. It uses rigid conductors

that are not under constant strain this makes low probability of breakage and increases the

reliability of the substation [24]. The rigid bus requires less area than a strain bus because

of less clearance of phase to phase.

Now to select the busbar construction among listed three options, the researcher need to

consider the milestones considered in IEEE substation design which are reliability,

security, maintainability, cost, environmental impact and others. Low profile rigid bus

has better reliability because of less liable to breakage and less cost due to lower structure

because of these reasons the researcher selects low profile rigid bus type.

3.5 Selection of Busbar Type

Busbar materials are available in different kinds, construction types and different alloys.

Among the best conductors, aluminum and copper are commonly used in substations

because of their economical and mechanical advantage over others like gold and silver.

Copper conductors can be classified into three based on their fabrication system as hard

drawn, medium-hard drawn and soft drawn copper. Hard drawn coppers are drawn to

wire without heat treatment have high strength and low elongation property which makes

it perfect for high tension lines. Soft drawn copper conductors are fabricated with

annealing process. They have a property of flexibility and mostly they are used for

grounding purposes.

Aluminum is using as a standard for transmission and substation bus bar works because it

has less weight and cost than copper. Even though aluminum has less tensile strength and

conductivity than copper, it is possible to amend this by increasing the cross-sectional

area of aluminum [25].


31

The selection of busbar conductors depends not only on the characteristics of electrical

conductivity but also on mechanical strength of the conductor. Therefore alloy of

aluminum hardened by different tempering processes commonly used in substation

because it has better strength than pure aluminum. Among them, the most commons ones

are 6063-T6, 6101-T6. They have 53 to 55% conductivity of IACS and have good

advantage of mechanical strength. If more mechanical strength required we can use 6061-

T6 with conductivity 40% of IACS but stiffer [26].

For this thesis, the researcher selected a 6106-T6 aluminum tube which has the maximum

conductivity (55% IACS) and all-aluminum conductors to connect the busbar to

substation equipment.

3.6 Busbar sizing

The busbar sizing will include determining busbar size for normal load and short circuit

current and calculating maximum corona. To calculate the above-mentioned

specifications, it is required to consider the conductor ampacity, the maximum expected

fault current, the operating voltage and fault clearing time.

Based on the load calculations, the total load is 50 MVA and the voltage that will be

stepped down is from 132 kV to 33 kV.

Using the above information the transformer secondary full load current will be:-

(3.1)

The next step is to find the fault current by assuming the impedance value based on the

standard of IEEE C57.12.10-2010 which deals with requirements for liquid-immersed

power transformers. Based on the above-mentioned standard the impedance percentage

value of oil-immersed power transformer should be less than 5. The researcher assumed

the percentage impedance 4%.


32

Now calculating short circuit current on secondary

( ) 1

%

00sc

Rated Current xI

Z (3.2)

(874.77) 100

4scI

= 21,869.25A

From ampacity Table in annex 1, that relates the cross-sectional area and wall thickness

of aluminum tubular bus with current carrying capacity, the standard size near to

874.77A is 981A which has pipe size, D=1.5 inch or 38.1mm, outside diameter, OD=1.9

in or 48.26mm and wall thickness t=0.145 in or 3.68mm.

For the HV side

50000

218.693 132

I A

218.69 100

5,467.254

SCI A

Using the ampacity Table of annex 1, the cross-sectional area of the aluminum tabular

bus that relates to the above mentioned rated current is SPS size is 25.4mm, OD 33.4mm

and wall thickness 3.38mm.

But according to the IEEE standard 605TM

-2008, the design should be confirmed using

the equation of heat balance because the current carrying capacity of the conductor is

limited by the conductor’s maximum operating temperature.

2

s c r condI RF q q q q (3.3)

Where

I is the current through the bus conductor, A

R is the direct-current resistance at the operating temperature, Ω/M

F is the skin-effect coefficient

qs is the solar heat gain, W/m

qc is the convectional heat loss, W/m

qr is the radiation heat loss, W/m

qcond is the conductive heat loss, W/m


33

Now the target is to confirm that the LV and HV side busbars of type 6106-T6 aluminum

alloy with the above-mentioned size can carry 875A and 219A current under normal

conditions and 24,525A and 5,468A under fault conditions by considering the working

temperature and the temperature rise during energy loss.

The calculation below is for the 33kV busbar.

c r cond sq q q qI

RF

(3.4)

According to IEEE standard 605TM

-2008 conduction is a minor method of heat transfer

because the contact surface is usually very small there it is usually neglected in bus-

ampacity calculations. For tubular bus conductor with 50 Hz the skin effect is assumed as

equal to one.

The resistance of aluminum alloy computed as:

6

2

1.724 10 0.004031 20

61c

CR T

C A

(3.5)

Where:

R is the DC resistance (Ω/m)

C is the conductivity as % IACS

Ac is the cross-sectional area, m2

T2 is the conductor temperature, °C

Here the selected bus 6106-T6 aluminum alloy has a conductivity of 55% IACS at

continuous rated temperature 90oC

2 2( 2 )

4cA D D t

(3.6)

2 2[(0.0381) (0.0381 2 0.00368) ]4

x

3.979x10-4

m2


34

Therefore the DC resistance will be:-

6

4

1.724 10 0.00403 551 90 20 84.5 /

55 3.979 10 61

xR x m

3.6.1 Convectional heat loss

Heat transfer of a cylindrical shape conductor with one inch diameter and wind 0.6 m/s

and 1 atmospheric pressure can be estimated as:

0.43.56cq D A T (3.7)

Where

qc is the convectional heat loss, W/m

A is surface area by unit length m2/m

D is diameter of the busbar

T is the temperature difference between the conductor and the

atmospheric area

The surface area with 0.038100 m diameter and one meter length can be calculated

A DL (3.8)

20.38 1 1.194 /A m m

0.43.56 (0.38) 1.194 (90 40) 312.98 /cq W m

3.6.2 Radiation heat loss

Emissivity is the ratio of power radiated by a material body to the power radiated by a

black body at the same temperature. According to IEEE Std 605TM

-2008 the emissivity

and absorptivity of a bus conductor taken as equal though they have different energy

spectra. The values of emissivity and absorptivity for copper and aluminum are listed in

the table below.


35

Table 5 Values of emissivity and absorptivity for copper and aluminum

Emissivity, absorptive of copper and aluminum

Copper Aluminum

Clean mill finish 0 0.1

Light tarnish (recent

outdoor installation or

indoor)

0.3 - 0.4

0.2

After extended outdoor

exposure

0.7 - 0.85 0.3 -0.5

Painted black 0.9 - 0.95 0.9 - 0.95

For this thesis, 0.5 emissivity is used.

8 4 45.6697 10 [( 273) ( 273) ]r c aq A T T

(3.9)

Where

A: surface area by unit length

ε : emissivity

Ta: ambient temperature

Tc: Conductor temperature

8 4 45.6697 10 0.5 1.194 [(90 273) (40 273) ] 262.83 /rq W m

3.6.3 Solar heat gain

Solar heat gain refers to the increase in thermal energy of a conductor by absorption of

solar radiation and it can be estimated using the following formula.

' ' sin( )s sq Q AK (3.10)


36

Where

qs=Solar heat gain by the conductor length

ε'=Coefficient of solar absorption

A'=The project area of the conductor

Qs=The total solar radiated heat on a surface normal to sun's ray

K=Heat multiplying factor for higher altitude

θ=The effective angle of incidence of sun

Before calculating solar heat gain, it is needed to determine the effective angle of

incidence of the sun for the site where the substation is going to be built

1cos [cos cos( )]c c lH Z Z (3.11)

Where:

Hc is the altitude of the sun for the latitude of 11.36o at noon time which is 10.95

o

Zc is the azimuth of the sun for the latitude of 11.36o at noon time which is 280.07

o

Zl is the azimuth of conductor line and the assumption of laying is from North to south

orientation which will have 0 degree

Therefore the angle of incidence will be:-

1cos [cos(10.95)cos(280.07 0)] = 80.1

o

ε' the solar absorption of weathered aluminum =0.5

From the table of total solar radiated heat on a surface normal to sun's ray for Hc 10.95

and Zc 280.07 is 1153.8w/m

Using the above information the solar heat gain will be:-

0.5 1153.8 0.0381 1.036 sin(80.1) 22.4 /sq W m

Now it is possible to calculate the current carrying capacity of the conductor


37

6

312.98 262.83 22.4

84.5 10I

2556.87I A

As the above calculation shows, 6106-T6 aluminum alloy with pipe size, D= 0.0381m,

outside diameter, OD=0.048260m can carry the full load under normal condition with its

maximum operating temperature. But it needs to be checked for carrying short circuit

current which is 21,869.25A.

6

10

20 (15150 / )10 1/ log (

20 (15150 / )

o

f

o

i

T GI C A t

T G

(3.12)

Where

I is the maximum allowable root-mean-square (RMS) value of fault current,

A is the conductor cross-sectional area, mm2

G is the conductivity in percent International Annealed Copper Standard (IACS)

t is the duration of fault, sec

Tf is the allowable final conductor temperature, °C

Ti is the conductor temperature at fault initiation, °C

C is constant 2.232x10^-4 for aluminum

4 6

10 0

1 250 20 (15150 / 55)2.232 10 10 1194 log ( )

20 90 20 (15150 / 55)

o o

oI

24227.49I A

Now the researcher confirmed that the value selected from the current carrying

capacity table can carry the normal as well the short circuit current by calculation.


38

3.6.4 Voltage gradient

Corona develops when the voltage gradient at the surface of the conductor exceeds the

dielectric strength of the air surrounding the conductor and ionizes the air molecules.

From Peek's experiment [27] the corona and electromagnetic interference (EMI) which is

caused by the corona occurs at a voltage gradient from 10kV/cm to 30kV/cm.

Corona creates electromagnetic interference to transmission systems including relay

communication for protection purpose, corona with water droplets create nitric acid

which can affect insulator and bring premature mechanical failure. Corona creates light

emission which is equivalent to the spectrum of sunlight and this can increase the

temperature on the bus.

Even though difficult to remove completely the effect of corona, it is possible to improve

its effect by increasing phase spacing, conductor diameter or shape, distance from the

ground or applied voltage. Here the goal is to reduce the corona below the allowable

voltage gradient by using the above-mentioned techniques. For corona free operation, the

maximum surface voltage gradient of the bus conductor Em should be less than the

allowable surface voltage gradient Ec [28].

First, we need to calculate the corona onset gradient voltage for the site and then calculate

the maximum voltage gradient for the selected bus size using Peek's formula [27] and

then check that the gradient voltage is lesser than the corona onset gradient voltage.

1c o a

a c

CE mE D

D r

(3.13)

Where

Ec is on set gradient voltage

m is air density factor and its value for the worst case is 0.85

Eo and C are empirical constants and their value 21.1kV/cm and 0.301cm-1

respectively.


39

rc is conductor outside radius

Da is relative air density

But first, it is needed to calculate air density which is a function of altitude and

temperature.

273

1273 10

oa

T AD

T

(3.14)

Where

T is ambient air temperature 19.6oC

Ta is air temperature used in determining above listed constants which is 25oC

A is the altitude in Km which is 1.797Km

273 25 1.797

1 0.835273 19.6 10

aD

Now the onset gradient voltage will be

0.3010.85 21.1 0.835 1 18.15 /

0.835 2.413cE kV cm

Here it shows that the corona onset value for the site is 18.15kV/cm

The formula to find the maximum voltage gradient for the rigid bus is given as below,

2

em a

e

hE E

dh

(3.15)

Where

Em is maximum voltage gradient at the surface of the conductor

he is the center distance from ground, cm

d is conductor diameter cm


40

Ea is the average voltage gradient at the surface of the conductor

The average voltage gradient Ea calculated as:-

1

4ln( )

2

a

VE

d h

d

(3.16)

Where

V1 is the phase to ground voltage multiplied by 110%

For the three-phase configuration, the center conductor has a gradient of approximately

5% higher than the outside conductors. And V1 can be approximated by 110% of a

nominal operating line to ground voltage [23].

1 (33 / 3) 110% 20.96V KV kV

d is conductor diameter which is 3.81cm and h is assumed 400cm.

20.961.76 /

3.81 4 500ln

2 3.81

a

KVE kV cm

The maximum voltage gradient will be

5001.76 1.77 /

3.81500

2

mE kV cm

Based on the above calculations, the maximum voltage gradient is less than on set

gradient voltage.

For 132kV bus bur which has SPS size are 25.4mm, OD 33.4mm and wall thickness

3.38mm. As do for 33kV BB the resistance of aluminum alloy can be computed using

equation 4.5.


41

6

2

1.724 10 0.004031 20

61c

CR T

C A

Here also the selected bus type 6106-T6 aluminum alloy have a conductivity of 55%

IACS at continuous rated temperature 90oC.

2 2( 2 )4

cA D D t

2 2 4[(0.0254) (0.0254 2 0.00338) ] 2.3338 104

Therefor the DC resistance will be:-

6

4

1.724 10 0.00403 551 90 20 168.47 /

55 2.3338 10 61

xR x m

The convectional heat loss estimation calculated as:

0.43.56cq D A T

The surface area with 0.0254m diameter and one meter length can be calculated

A DL

20.0254 1 0.0798 /m m

0.43.56cq D A T

0.43.56 (0.0254) 0.0798 (90 40) 61.73 /cq W m

Radiation heat loss

As do for 33Kv busbar, the radiation heat loss also can be calculated using the following

formula.

8 4 45.6697 10 [( 273) ( 273) ]r c aq A T T


42

Where

A: surface area by unit length

ε : emissivity

Ta: ambient temperature

Tc: Conductor temperature

8 4 45.6697 10 0.5 0.0798 [(90 273) (40 273) ] 17.57 /rq W m

Solar heat gain

Since the 33kV and 132kV busbar have a different diameter of conductor, it is necessary

to calculate the solar heat gain for 132kV busbar but using the same value for the

effective angle of incidence of the sun.

' ' sin( )s sq Q AK

0.5 1153.8 0.0254 1.036 sin(80.1) 14.95 /sq W M

Now it is possible to calculate the current carrying capacity of the conductor

6

61.73 17.57 14.95

168.47 10I

618.03I A

Based on the above calculations the selected busbar with type 6106-T6 aluminum alloy,

pipe size 25.4mm, outside diameter 33.4 and wall thickness 3.38mm is suitable for the

proposed substation but it needs to be checked for carrying short circuit current which is

21,869.25A by the equation 4.12.

4 6

10 0

1 250 20 (15150 / 55)2.232 10 10 79.79 log ( )

20 90 20 (15150 / 55)

o o

oI

24227.49I A


43

Now the researcher confirmed that the value selected from the current carrying

capacity table can carry the nominal as well the short circuit current by calculation.

Similarly, as do for 33kV busbar, the voltage gradient can be calculated as follow

. 1c o a

a c

CE mE D

D r

The relative air density as calculated in equation 4.14 and its value is 0.835

Now the onset gradient voltage will be

0.3010.85 21.1 0.835 1 18.79 /

0.835 1.67cE kV cm

Here it shows that the corona onset value for the site is 18.79kV/cm

Using equation 4.15 and 4.16 the average and maximum voltage gradient on the busbar

can be calculated as ;

83.83

10.62 /2.45 4 600

ln2 3.81

aE kV cm

Having Ea the average voltage gradient, the maximum voltage gradient calculated as;

60010.62 10.64 /

2.45600

2

mE kV cm

Based on the above calculations, the maximum voltage gradient is less than on set

gradient voltage.

3.7 Selection of Transformer

The detail technical specification for power transformer is based on the IEEE standard

C57.12.10TM

-2010 which deals on requirements for liquid immersed power transformer.


44

As calculated and selected in chapter 2, the transformer rating is 50 MVA with an equal

MVA rating for backup and 132/33kV ratio. But the selection of the size is by

considering ONAN cooling system. If ONAF system of cooling selected, the rating will

be decreased by 133%. This means if ONAF selected 37.5MVA rating transformer will

be enough to power the load of the enterprise.

Based on the above-mentioned standard, the cooling system must be continuous with the

average winding temperature not exceeding 65 degree centigrade and hottest spot

temperature rise should not exceed 80 degree centigrade. The oil temperature rise shall

not exceed 65 degree centigrade when measured near the top of the tank.

According to the IEEE Std C57.12.10TM

-2010, the preferred tap changer is on-load tap

changer (OLTC) which will be connected in low voltage winding to provide

approximately ±10% automatic regulation in approximately 0.625% with 16 steps above

and 16 steps below rated voltage.

In the above mentioned IEEE standard, the percentage impedance voltage of self-cooled

transformer is given in a table that base on the high voltage BIL in kV. According to this,

the percentage impedance voltage should not be greater than 7%, winding resistance not

greater than 10% and power factor not less than 0.91.

Since one 50 MVA transformer is connected for the substation there might not be a

problem of selecting vector group but since mostly generators are connected YNd1 to

protect the generators from earth fault, DYn11 will be preferred for transformers so that

to remove the 30 degree lagging phase angle difference created.

Based on the standard the transformers need to have accessories like indicators of oil

level, oil temperature, winding temperature, pressure vacuum gauge, pressure-vacuum

bleeder valve, pressure relief device, drain and filter valves, sudden pressure relay, alarm

contacts and lifting facility.


45

Table 6 Specification for transformers

S/No Description Value

1 Type Three-phase two winding

2 Rated voltage 132/33kV

2 Cooling system ONAN/ONAF

3 Rated capacity 37.5/50 MVA

4 System frequency 50 Hz

5 Impedance 7% at 25.2 MVA

6 Connection DYn11

7 Resistance 10% at 25.2 MVA

8 Service Outdoor

9 Duty Continuous

3.7.1 Voltage Drop at Transformer

The power transformer output voltage may get down due to leakage reactance and

winding resistance but it should not go above 6% which is the permitted value mentioned

in the IEEE guide for transformer loss measurement under the standard of IEEE Std

C57.123-2010 [29].

The voltage drop in the transformer calculated as below:-

1

2 2 2cos ( sin ) /100%U R X

(3.17)

Where ∆U is percentage voltage drop at full load

X is percentage Leakage reactance

R is percentage resistance

1

2 2 210 0.91 (7 0.41) /100%U

∆U=0.094%

The voltage drop is below the permissible value recommended by IEEE.


46

3.8 Transmission Line

The existing 132kV transmission line that installed from Tis Abay generation stations to

Bahir Dar substation II which has 30Km length will be replaced by LILO connection.

The new double-circuit 132kV transmission line will be lined from Tis Abay generation

stations to AMIMTDE substation, 38.5km and from AMIMTDE Substation to Bahir Dar

Substation 8.5km, .

The existing overhead transmission line type is AAAC, area 150mm2

stranding wire

number 19 and wire diameter 3.18 mm, overall diameter 17.4mm, maximum DC

resistance at 20 oC 0.183 ohm/Km, stranding factor 0.7577 and ampacity 300A [30].

Therefore not to replace all transmission lines with a new one, we need to consider the

existing transmission line and check for loading current capacity.

To check the cable size, we need to calculate the full load current and use de-rating

factors to increase the margin of safety if they operated more than the maximum power

dissipation.

350 10218.69

3 132full load

KVI A

KV

(3.18)

From IEC 60287 the de-rating factor by considering 30 degrees centigrade of

atmospheric temperature is 0.89. Therefore the current that will flow through the feeder

will be;

ratedII

Derating factor (3.19)

218.69245.72

0.89

AI A (3.20)

The existing AAAC cable with area 110.9mm2 and ampacity of 300A can be used as

overhead transmission lines.


47

3.9 Selection of Isolators

Isolators are mechanical switch used to make and break electric circuits. Isolators used at

offload conditions because they don’t have arc quenching mechanisms or the arc

quenching that added used to interrupt a very small amount of charging or magnetizing

current [31].

Isolators can be operated automatically or manually. Isolators are designed thermally

stable for their continuous current-carrying and for specified short time fault current

rating.

There are three main types of isolators; pantograph, double breaks and single break. The

pantograph isolator is single break and vertical connection type isolator mounted on a

post insulator operated through a rod insulator. Pantograph permit the design of modern

switchgear installations with minimum space requirements but its price is more expensive

than others. Because of this it is mostly used in electric train to collect power through an

overhead tension wire and in substations which has a space problem.

A double break isolator consists of three loads of post insulators. The middle insulator

holds a flat male or tubular contact that can be turned straightly by a spin of middle post

insulator that can be done manually or with motor. In single break isolator the arm

contact is separated into two elements. The first arm contact holds male contact and the

second arm contact holds female contact. The arm contact shifts because of the post

insulator rotation upon which the arm contacts are fixed.

Earthing switch should be mounted in three of the isolator types which will be engaged

when the isolator at off condition. The earthing arms are interlocked with the main

isolator moving contacts that will be closed only when the primary (feeder) contacts of

the isolator in open condition.

3.9.1 Allowable continuous current of isolators

The allowable continuous current of isolators at specific ambient temperature is the

maximum alternating current in RMS amperes at rated frequency, that the isolator will


48

carry without exceeding the allowable maximum temperature for any of its parts [23], as

listed in the table below.

Table 7 Temperature limit for Isolators

Isolator Part Allowable

maximum

temperature,

θmax(°C)

Limit of observable

Temperature rise at

rated current, θr(°C)

Copper or copper alloy 75 33

Copper or copper alloy to silver or

silver alloy, or equivalent.

90 43

Silver, silver alloy, or equivalent. 105 53

According to IEEE std 37.30.1-2011, the allowable current can be calculated as follows:

0.5

max aa r

r

I I

(3.21)

Where

Ia is allowable continuous current at ambient temperature.

Ir is rated continuous current

θmax is allowable maximum temperature (°C) of switch part from the above table

θa is the ambient temperature

For 132kV which has rated current of 218.69A

75 40

218.69 ( ) 231.9433

o o

a o

c cI A A

c

For 33kV which has rated current of 874.77A

75 40

874.77 ( ) 927.7833

o o

a o

c cI A A

c


49

3.9.2 Rated short-time withstand current

The rated short-time withstand current has two associated ratings: rated peak withstand

current, which is a measure of the switch’s ability to withstand the magnetic forces

associated with a short circuit and rated short-time (symmetrical) withstand current

duration, which is a measure of the switch’s ability to withstand the heat generated by a

short-time current.

The relationship between peak withstand current and short-time (symmetrical) withstand

current is based on an approximate X/R of 17 for 60 Hz applications and X/R of 14 for 50

Hz applications. This leads to a peak current to RMS symmetrical current ratio of 2.6 for

60 Hz and 2.5 for 50 Hz [23].

Table 8 Specification of Isolator

S/No Description Requirements for 132kV

Isolator

Requirements for

33kV Isolator

1 Type of isolator Single break with manual

and electrical operated

Single break with

manual and

electrical operated

2 Rated power frequency 50Hz 50Hz

3 Rated voltage 132kV 32kV

4 Rated maximum voltage 145kV 36kV

5 Allowable continuous current 232A 928A

5 Rated short time withstand current 580A 2320A

6 Rated power frequency withstand

voltage for a duration of 10 seconds 170kV 70kV

7

Rated lightning impulse withstand

voltage for peak value of 1.2/50 μs

positive and negative impulses.

550kV 170kV


50

3.10 Selection of Circuit Breakers

Circuit breakers used for making and breaking electric circuits under normal or during

abnormal conditions by continuously monitoring the current that flows through it. They

can be monitored manually or from remote.

Circuit breaker that used in substations broadly classified as indoor or outdoor. Their

main differences are their enclosure or the outdoor have higher number of IP otherwise

their internal current-carrying parts, the interrupting chambers and the operating

mechanisms are the same. Most of the time when the voltage is higher outdoors type is

preferred to be safe from fire hazard [32].

From physical structure design, outdoor circuit breakers can be identified as dead tank or

live tank. In live tank circuit breakers, the switching unit is located in an insulator busing

which is live at line voltage were as in dead tank the switching unit is located in a

metallic enclosure which is kept at ground potential. Even though using dead tank type

circuit breakers able to mount multiple low voltage bushing type current transformers to

installed at both the line and load side, live tank circuit breaker type is preferred by IEC

standard followers because of less in price, less mounting space and less interrupting

fluid [32]. Therefore, outdoor circuit breakers with live tank construction types are

selected for the thesis.

Table 9 Specification for Circuit Breaker

S/No Description Requirements for

132kV circuit

breaker

Requirements for

33kV circuit

breaker

1 Operating mechanism Tri pole Tri pole

2 Switching device type Live tank Live tank

3 No of pole Three Three

4 Rated service voltage 132kV 33kV

5 Rated Current 300A 1250A

6 Rated short circuit current 145kV 36kV


51

7 Insulating media SF6 SF6

8 Short circuit withstand current

duration

0.5 sec

0.5 sec

9 Insulation level

9.1 Power frequency withstand (kV RMS

for 1 min)

170kV

70kV

9.2 Impulse withstand (1.2/50µsec) kV

peak

650kV

170kV

3.11 Selection of Instrument Transformer

Instrument transformer used in substation for measuring electrical parameters which will

be used for control and protection systems. The level of voltage and current in the

substation is very high because of this; it is beneficiary to step down their value to use

low voltage and current transformers so that to decrease the cost of control and protection

systems and low power consumption of the above-mentioned instruments. The level is

usually 5 or 1 A for current transformers and 110 V for voltage transformers [33].

According to IEEE Std C57.13TM

-2016 instrument transformers confirming the standard

shall be suitable for operation at their thermal ratings provided that the altitude doesn't

exceed 1000m above sea level. If the altitude exceeds, it gives a value of dielectric

strength correction factor.

Table 10: Dielectric strength correction factors for altitudes greater than 1000m

Altitude

(m)

Altitude correction factor for

dielectric strength

1000 1

1200 0.98

1500 0.95

1800 0.92

2100 0.89


52

2400 0.86

2700 0.83

3000 0.8

3600 0.75

4200 0.7

4500 0.67

The other points that IEEE Std C57.13TM

-2016 stress on instrument transformers are

accuracy class. Accuracy classes for metering are based on the requirement that the

Transformer Correction Factor (TCF) of the voltage transformer or the current

transformer shall be within the specified limits when the power factor (lagging) of the

metered load has any value from 0.6 to 1.0. The following formula is used to find the

correction factor for the worst case, 0.6 lagging power factor [33].

For current transformer 2600

TCF RCF

(3.22)

For voltage transformer 2600

TCF RCF

(3.23)

Where

TCF is a transformer correction factor

and are the phase angle in minutes for current and voltage transformer

respectively

RCF Ratio Correction Factor

The Standard accuracy class for metering service and corresponding limits of transformer

correction factor and ratio correction factor [0.6 to 1.0 power factor (lagging) of metered

load is listed in the Table 11.

.


53

Table 11: The standard accuracy classes

Metering

accuracy

class

Voltage

transformers

(at 90% to 110%

rated voltage)

Current transformer

Min

Max

At 100% rated

current

At 10% rated

current

At 5% rated

current

Min Max Min Max Min Max

0.15S - - 0.9985 1.0015 - - 0.9985 1.0015

0.15 0.9985 1.0015 0.9985 1.0015 - - 0.9970 1.0030

0.15N - - 0.9985 1.0015 0.9970 1.0030 - -

0.3S - - 0.9970 1.0030 - - 0.9970 1.0030

0.3 0.9970 1.0030 0.9970 1.0030 0.9940 1.0060 - -

0.6 0.9940 1.0060 0.9940 1.0060 0.9880 1.0120 - -

1.2 0.9880 1.0120 0.9880 1.0120 0.9760 1.0240 - -

3.11.1 Current transformer

Current transformers installed in such a way that the conductor act as a primary winding

and the induced current in the CT passes through ammeter with a certain predetermined

ratio. According to IEEE Std C57.13TM

-2016, the main parameters for the selection of

current transformers are operating voltage level, accuracy class, and thermal short time

rating, impulse withstand voltage during short circuit and normal operating current.

Table 12: Specification for current transformer

S/No Description Requirements for

132kV CT

Requirements for

33kV CT

1 Type Outdoor oil

immersed.

Outdoor oil

immersed.

2 Nominal system voltage 132kV 33kV

3 Maximum system voltage 145kV 36kV


54

4 Frequency 50Hz 50Hz

5 Phase Single Single

6 Current rating ratio 250:5 1000:5

7 Rated short circuit maximum current 40kA 100kA

8 Power frequency withstand voltage

(kV RMS for 1 min)

275kV 70kV

9 Impulse withstand voltage

(1.2/50µsec) kV peak

650kV 170kV

3.11.2 Potential transformer

Potential transformers are used to step down the voltage to a lower value which enables

us to measure the high voltage with low voltage instruments by isolating them from high

voltage. The scale of the voltmeter is calibrated according to the primary line voltage.

The primary of the PT is connected across the phases of the high voltage and the

secondary is earthed to reduce high voltage if a loose connection persists.

According to IEEE Std C57.13TM

-2016, the main parameters for selection of potential

transformers are rated voltage, rated maximum voltage, rated secondary voltage, power

frequency withstand voltage and impulse withstand voltage.

Table 13 Specification for potential transformer

S/No Description Requirements for 132kV

PT

Requirements for

33kV PT

1 Type Outdoor, single phase Outdoor, single

phase

2 Rated voltage 132/ kV 33/ kV

3 Rated maximum voltage 145kV 36kV

4 Frequency 50 Hz 50 Hz

5 Power frequency withstands

voltage for 1 min for

primary winding.

170kV 70kV


55

6 Power frequency withstands

voltage for 1 min for

secondary winding.

2kV 2kV

7 Impulse withstand voltage 650kV 170kV

3.12 Surge Arrestor Selection

Surge arrestors are designed to limit the voltage surges power circuits by passing surge

discharge current. They establish a baseline of transient overvoltage above which the

arrester will operate. When a transient overvoltage appears at an arrester location, the

arrester conducts internally and discharges the surge energy to ground. Once the

overvoltage is reduced sufficiently, the arrester seals off, or stops conducting, the flow of

power follow current through itself and the circuit is returned to normal.

Transformers, regulators, and other substation equipment are sensitive to transient

overvoltage. For the highest degree of equipment protection, surge arresters should be

installed closely to the equipment being protected. There are different kinds of surge

arrestors like rod gap arrester, horn gap arrestor, multi-gap arrestor, expulsion type

lightning arrester and metal oxide surge arrestor but metal oxide gap arrestor has the

following advantages over the others.

1. It eliminates the risk of spark over and also the risk of shock to the system when

the gaps break down.

2. It eliminates the need of voltage grading system.

3. At the normal operating condition, the leakage current in the ZnO is very low as

compared to other diverters and there is no power follow current in ZnO diverter.

4. It has high energy absorbing capability.

5. ZnO diverters possess high stability during and after prolonged discharge.

6. In ZnO diverter, it is possible to control the dynamic overvoltage in addition to

switching surges this makes it economic insulation coordination.

Because of the above advantages metal oxide surge arrestors are selected for this thesis.

The ratings of the arrestor are by basing on IEEE Std C62.11™-2012.


56

Table 14 Ratings for surge arrester

S/n Description Ratings for

132kV

Ratings for

33kV

1 Type Outdoor Outdoor

2 Maximum Continuous Operating

Voltage (MCOV)

106kV 29kV

3 Rated Frequency 50 Hz 50 Hz

4 Nominal Discharge Current 10kA 10kA

5 Rated Short-time- Current 31.5kA 25kA

6 Power Frequency Withstand

Voltage for 1 min

170kV 70 kV

7 Impulse Withstand Voltage (1.2/50

μsec) kV Peak

650 kV 170 kV

8 High current 4/10 impulse withstand

value

100kA 100kA

3.13 Lightning Stroke Shielding

Lightning is a discharge of atmospheric electricity from one cloud to another or from a

cloud to the earth. A single bolt of lightning can generate more than 27000-degree

centigrade heat that can completely damage the substation and its equipment completely

[34]. Therefore, there should be a protection system of direct stroke lightning shielding.

There are different types of stroke like stroke within clouds, between separate clouds,

stroke to tall structures and stroke that terminate on the ground. Among them, the one

that can damage substation is the last one which has positive and negative terminating on

the ground.

There are two steps of process for stroke development. The first step is ionization of air

surrounding the charge center which propagates the charge from the cloud into the air and

its magnitude is in the order of 100A. The second step is return stroke which is an

extremely bright streamer that propagates upward from the earth to the cloud which has

an average value of about 24,000A [35].


57

There are different methods of design of lightning stroke shielding which are mentioned

in IEEE Std 998TM

-2012. Among them fixed angle, empirical and electro-geometric are

popular lightning stroke shielding methods.

The fixed angle design method uses vertical angles to determine the number, position and

height of shielding wires. The physical area of the substation determines the angle alpha

and beta which is shown in figure 13. This method is easy to do and to analysis but

inaccurate for long heights, ignores varying stroke current magnitudes and no practical

failure analysis for uncovered regions.

The empirical method is based on the experiment which is done by creating a spark to

simulate lightning and creating a graph by considering 0.1 and 1 percent exposure. The

graph has the ratio of the distance between the mast and equipment to the height of the

mast vertically and distance of the substation equipment to a height of the mast and by

fixing the distance between the mast and equipment the graph gives us how much should

be the height of the mast. But this method has a disadvantage of limitation of exposure to

one and 0.1 percent only and no practical failure rate analysis for uncovered regions.

Electro-geometrical or rolling sphere method which was developed in 1971 by Edison

Electric Institute. It is based on the electro-geometrical model, which strongly considers

the physics of natural lightning [36].


58

Figure 13: Protection of substation using mast with fixed angle method

Figure 14: Empirical Curve with 0.1 and 1 percent exposure

Rolling sphere method recognizes the attractive effect of the lightning rod is a function of

a striking distance which is determined by the amplitude of the lightning current. It has an

advantage of no limitation to exposure rate.


59

The strategy of rolling sphere method is using an imaginary sphere of radius S over the

surface of a substation. The sphere rolls up and over lightning masts, shield wires,

substation fences, and other grounded metallic objects that can provide lightning

shielding. A piece of equipment is said to be protected from a direct stroke if it remains

below the curved surface of the sphere.

To design the lightning protection we need to calculate surge impedance Zs allowable

stroke current Is and striking distance Sf.

0.658f sS KI (4.24)

K is a coefficient to account for different striking distances to a mast, a shield wire, or the

ground plane. It has a value of k = 1 for strokes to wires or the ground plane, and a value

of k =1.2 for strokes to a lightning mast.

Stroke current Is is associated with BIL of the equipment. Since a flash or an arc which is

under the BIL cannot create flashover or it goes to ground through the enclosure.

2.2

s

s

BILI

Z

(3.25)

Where

Is is allowable stroke current in kA

BIL is basic impulse level in V

Zs is the surge impedance of the conductor in ohm

According to IEEE Std C62.22, bus insulators are usually selected to withstand BIL other

substation equipment especially those that have a small amount of BIL may not be

protected by the design stoke current Is but they will be protected by the surge arrestors.

60ln(2 ( ) / ( ))sZ h e r c

(3.26)

Where

h(e) is height of equipment in meter

r(c) is radius of the conductor in meter

360ln(2(8) / (8 10 ) 456sZ


60

Table 15 Basic Data to calculate stroke distance

Description 132kV busbar 33kV

busbar

Voltage Level 132kV 33kV

BIL 650kV 170kV

Height 8 meter 6 meter

Diameter of conductor used for shielding 8mm round copper conductor

Maximum height for mast (assumption) support point 18 meter

Now the stroke current Is will be:

2.2 650000

3.14456

s

VI KA

Using the stroke current we can find the radius of the sphere as:

0.658 1.2 3.14 20.2S m

Figure 15: Lightning protection using rolling sphere


61

Figure 16: Substation protected from lightning

3.14 Earth Mat Design

One of the major issues during substation design is safety. To ensure safety for operators,

one of the tasks is properly designed earth mat. According to standard IEEE std 80-2013,

earth mat is defined as a solid metallic plate or a system of closely spaced bare

conductors that are connected to and often placed in shallow depths above a ground grid

or elsewhere at the earth’s surface, in order to obtain an extra protective measure that

minimize the danger of the exposure to high step or touch voltage in a substation or

critical operating area which are frequently used by people. Grounded metal frameworks,

placed on or above the soil surface, or wire mesh placed directly under the surface

material are common forms of a ground or earth mat [37].

The objectives of earth mat design are:

1. To create a means to carry electric currents into the earth under normal

and fault conditions without exceeding any operating and equipment limits

or adversely affecting continuity of service.


62

2. To protect personnel and equipment exposed to a potential gradient in the

vicinity of grounded facility.

3. To provide a low resistance path for lightning and over-voltage conditions.

4. To protects the buildup of electrostatic charges which can cause spark that

can make ignition to flammable gas or liquids.

3.14.1 Resistance of human body

Dalziel approximated human body resistance, from one hand to both feet or from one

foot to the other one from 500 to 3000 ohm, but it will decrease when damage or

puncture of the skin occurs at the point of contact. IEEE std 80-2013 represents human

body resistance from hand to feet and also from hand to hand as 1000 ohm.

3.14.2 Data collection on soil resistivity

The first task in mat design is measuring the earth resistivity of the site where the

substation is going to erect. The soil has three components solid, liquid and gas. The solid

phase of normal soil usually includes mineral and organic matter and the liquid phase

containing the water solution and the gas phase is the air between the solid particles. The

conductivity of the soil strongly determined by the water content and the water state.

Earth resistivity is a measure of how much the soil resists the flow of current through it

[38].

Table 16 Range of earth resistivity

Type of earth Average resistivity (Ω-m)

Wet organic soil 10

Moist soil 102

Dry soil 103

Bedrock 104

The researcher has done the resistivity measurement using DSJD digital ground

resistance tester and the measuring method used is Wenner four-pin method. Four probes


63

drive to the ground with equal spacing between them, 4 meters and four of them at a

depth of 0.2 meters which is recommended by the manual, depth equals to spacing

divided by 20 and found 13.6 Ohms.

To find the resistivity the following formula will be used which is suggested by the

manual.

2 RL (3.28)

2 13.06 4 328.23 .m

Figure 17: Collecting data on site

The enterprise has 10.05 hectares of land for its factories but it uses 2.8 hectares only. It

keeps 7.25 hectares for future expansion. Therefore, the researcher assumes the enterprise

will use 100 x 100 meter (length x width) land for building the substation.

3.14.3 Effect of surface material

High resistive material usually gravel is spread on the earth's surface of the substation

area with 80 to 150 mm height to increase the contact resistance between the person and

the soil [37]. In addition to this, they keep the evaporation of the water content of top soil.

The value of the current that will go upward to the surface of the spread material depends


64

on the relative values of the soil and the surface material resistivity and thickness. For

AMIMTDE substation it is possible to use granite stone which can easily found around

Lake Tana which has 25-50 mm size and has a resistance of 1.5 x 106 Ω-m. According to

IEEE std 80-2013, the reduction effect is represented by Cs or it can be considered as a

corrective factor to compute the effective resistance of the surface material and it can be

computed by the following formula [37].

0.09 1

12 0.09

s

s

s

Ch

(3.29)

Where

Cs is surface layer derating factor

ρ is the resistivity of the earth beneath the surface material in ohm-meter

ρs is surface material resistivity in ohm-meter

hs is the thickness of the surface material in meter

6

328.230.09 1

1.5 101 0.36

2 0.08 0.09sC

3.14.4 Calculation of maximum grid current

When a ground fault occurs at a substation, the part of the fault that flows between the

grounding system and the surrounding earth is known as grid current. The magnitude of

the hazard voltage level is determined by this current. Calculation of the maximum grid

current helps full to determine the ground electrode size economically.

The grid current Ig of a substation can be expressed as a product of four factor as [39]

g f f p fI I D C S (3.30)


65

Where

Ig is grid current

If is the symmetrical fault current

Df is a decrement factor

Cp is corrective projection factor

Sf is current division factor

According to IEEE std 80, for safe design, it is needed to consider the worst condition

that makes the maximum value of grid current. In substations with three-winding

transformers, the worst case or the maximum grid current Ig may occur for a ground fault

on either the high or low side of the transformer; both locations should be checked. In

either case, it can be assumed that the worst fault location is at the terminals of the

transformer inside the substation, if the system contribution to the fault current is larger

than that of the transformers in the substation [37].

Therefor its symmetrical fault current with zero ground resistance is taken.

3

p

f

s t

VI

Z Z

(3.31)

Zs is the short circuit impedance of the system and calculated as:

2

p

s

sc

VZ

S (3.32)

3SC scS UI (3.33)

Isc is already calculated in equation 1.15 therefore

3 132 21869.25scS KV A

5000MVA

2132

3.485000

s

KVZ

MVA


66

Zt is the short circuit impedance of the transformer referred to the winding and calculated

as follow:

2

St sys

p

VZ Z

V

(3.34)

233

3.48 0.2175132

Now to find the symmetrical fault currents

13220.61

3 3.48 0.2175f

KVI KA

According to IEEE standard 80-2013, the design of a ground grid must consider the

asymmetrical current to do this, decrement factor Df takes into account to include the

effect of dc current offset. Therefore Df is taken from the Table 17 with the value of fault

duration as 0.5 seconds which is recommended by the above-mentioned standard and for

the least X/R ratio and its value will be 1.026.

Table 17 Typical values of decrement factor referred from IEEE std 80-2013

Fault Duration, tf Decrement factor, Df

Seconds Cycles X/R=10 X/R=20 X/R=30 X/R=40

0.00833 0.4165 1.576 1.648 1.675 1.688

0.05 2.5 1.232 1.378 1.462 1.515

0.1 5 1.125 1.232 1.316 1.378

0.2 10 1.064 1.125 1.181 1.232

0.3 15 1.043 1.085 1.125 1.163

0.4 20 1.033 1.064 1.095 1.125

0.5 25 1.026 1.052 1.077 1.101

0.75 37.5 1.018 1.035 1.052 1.068

1 50 1.013 1.026 1.039 1.052


67

CP, the corrective projection factor accounting for the future increase in fault current

during the life span of the substation but hence the capacity of the substation is designed

by considering the life span, the researcher assumed as 1.

Sf is the current division factor which is the fraction of total ground-fault current that

flows between the grounding system and the surrounding earth. It can be considered as 1

since the researcher is considering worst case.

Now the grid current Ig will be:

20.61 1.026 21.15gI KA KA

3.14.5 Diameter of grid conductor

The short time temperature rise in a ground conductor or the required conductor size as a

function of conductor current is calculated by the following formula below [37].

410 242 1084

ln242 40

G

c y r

IA

TCAP

t

(3.35)

Where

A is the conductor cross-section in mm2

IG is the rms current in kA which is already calculated as 21.15kA

Ko 1/αo or (1/αr) – Tr in °C

Tm is the maximum allowable temperature in °C

Ta is the ambient temperature in °C

Tr is the reference temperature for material constants in °C

αo is the thermal coefficient of resistivity at 0 °C in 1/°C

αr is the thermal coefficient of resistivity at reference temperature Trin 1/°C

ρr is the resistivity of the ground conductor at reference temperature Trin

μΩ-cm

tc is the duration of current in s and taken as 0.5 sec.

TCAP is the thermal capacity per unit volume from Table 1, in J/(cm3 · °C)


68

The thermal coefficient resistivity and resistivity of the ground conductor are found from

the table 18 [40].

From listed items in table three, the researcher selected copper commercial hard- drawn

because it has good conductivity which is 97% of IACS and since it is hard-drawn its

mechanical strength is greater than annealed soft drawn one.

4

21.15

3.4 10 242 1084ln

0.5 0.00381 1.78 242 40

KAA

A= 136.26 mm2

From the ground wire size chart, the standard size is near to the calculated value is 200

mm2 [41].

The diameter can be calculated as

2A

D

=15.96 mm

Table 18 Ground conductor sizing factor

Description

Material

conductivity

(% IACS)

αr factor

at 20 °C

(1/°C)

Koat 0

°C

(0°C)

Fusing

temperature

Tm

(°C)

Resistivity

at 20 °C

ρr

(μΩ-cm)

Thermal

capacity

TCAP

[J/(cm3 ·

°C)]

Copper,

annealed

soft-drawn

100 0.003 93 234 1083 1.72 3.4

Copper,

commercial

hard-drawn

97 0.003 81 242 1084 1.78 3.4

Copper-clad

steel wire 40 0.003 78 245 1084 4.40 3.8

Copper-clad

steel rod 17 0.003 78 245 1084 10.1 3.8

Aluminum-clad

steel wire 20.3 0.00360 258 657 8.48 3.561

Stainless steel,

304 2.4 0.001 30 749 1400 72 4


69

3.14.6 Grid resistance calculation

One of the parameters in substation design is to calculate accurately the grounding

resistance. The grounding resistance value needs to be around 1 ohm to minimize ground

potential rise that can cause dangerous touch and step voltage.

The grid resistance depends on the total area of the substation, the total length of earthing

conductors, the number of earthing electrodes and the type of soil. Depending on the

voltage level, NEC and IEEE recommended the following grid resistance value but in any

case, the grid resistance value should not be greater than 25 ohm [42].

1. Generating stations and substations less than 1 ohm

2. Primary distribution stations and huge industries 1-5 ohm

3. Equipment rated for 10kV and below, 400-440V substations 5 to 10 ohms.

There are different kinds of commercial software that are used to design and analyze

ground protection systems like CDEGS, SGW, SDWork station, WINIGS etc. But the

researcher got ETAP software.

ETAP is a full spectrum analytical engineering software company specializing in the

analysis, simulation, monitoring, control, optimization, and automation of electrical

power systems. The Ground Grid Systems module enables to quickly and accurately

design and analyze ground protection. It's soil analysis tools allow automatic generation

of a two-layer soil model from soil measurement data, based on the Wenner four-pin

method and use the IEEE 80-2000 substation grounding standard [43].

Design data

1. Soil resistivity = 328.23 Ω-meter

2. Resistivity of surface material = 1.5x10^6 Ω-meter

3. Depth of surface material = 150mm

4. Symmetrical short circuit current= 20.61kA

5. Grid current = 21.15kA

6. Diameter of earthing conductor =16mm

7. Duration of earth fault = 0.5second


70

8. Ambient temperature= 40 degree centigrade

9. Maximum allowable temperature of selected conductor=1084o

The researcher uses ETAP software to calculate the number of grid conductors by using

the above information with the intention of reducing the grid resistance around 1 ohm by

iterating the number of conductors that used or by changing the spacing distance between

the grid conductors and ground rods.

Figure 18: Input Data to ETAP software

Figure 19: Layout of ground grids and rods


71

The grid wire pattern and the rods that bring the resistivity below 1Ω are 10 conductors in

the X and 10 conductors in the Y directions and 8 rods at a depth of 2 meters using this, it

is possible to proceed to calculate the substation ground resistance.

The grid conductor combined length LT will be:

10 100 10 100 2000TL m (3.37)

8 2 16RL m (3.38)

The total length of buried conductor will be 2016m.

By using the above information the substation grid resistance will be:

1 1 11

20 201G

T

RL A h

A

(3.39)

where

RG= Grid resistance

P = Resistivity of the soil

LT=Total length of buried conductor

h= Height of rods

A= Area of the substation

1 1 1328.23 1 1.57

2016 20 10000 201 2

10000

GR


72

3.14.7 Maximum mesh voltage

Maximum mesh voltage is the highest possible touch voltage value that may be

encountered in the substation grounding system. In a safe grounding system, the

maximum mesh voltage has to be less than the tolerable touch voltage.

The maximum mesh voltage value is obtained as a product of the geometrical factor, Km;

a corrective factor, Ki, which accounts for some of the error introduced by the

assumptions made in deriving Km; the soil resistivity, ρ; and the average current per unit

of effective buried length of the grounding system conductor [37].

m i Gm

m

K K IE

L

(3.40)

Where

Em is maximum mesh voltage

p is the resistivity of the earth

Km is the geometrical factor

Ki is the irregularity factor

IG is fault current in kA

Lm is the effective burial length

Km is a function of number of parallel conductors N, their spacing D, diameter d and

depth of the grid burial h that mean Km=f(N, h, D, d) and it can be calculated as

following [44].

22 21 8ln ln

2 16 8 4 2 1

iim

h

D h KD hK

h d D d d K n

(3.41)

Where

D is the space between parallel conductors in meter which is 11.11 meter


73

d is the diameter of grid conductors in meter which is 0.016

h is the depth of grid conductors in meter which is 0.75 meter

Kii is corrective weighting factor adjusting for the effects of inner conductors on the

corner of the mesh but For grids with ground rods along the perimeter, or for

grids with ground rods in the grid corners, as well as both along the perimeter

and throughout the grid area the value of Kii is 1 [44].

Kh is the corrective weighting factor adjusting for the effects of grid depth

n is geometrical factor.

Kh can be calculated as

1h

o

hK

h (3.42)

Where

ho is grid reference depth and taken as 1

0.75

11

hK = 1.32

From IEEE Std 80, the geometrical factor n calculated as

a b c dn n n n n (1.1)

where2 c

a

p

Ln

L (1.2)

nb, nc and nd = 1 for a square grid

Lc is the total length of the conductor in the horizontal grid in meter and Lp is the

peripheral length of the grid in meter.

2 2000

10400

an n

The irregularity factor Ki used with the square geometry of the grid [37].


74

0.644 0.148 2.124iK n

Using the above information it is possible to calculate the geometrical factor Km.

211.11 2 0.751 11.11 0.75 1 8

ln ln2 16 0.75 0.016 8 11.11 0.016 4 0.016 1.32 2 10 1

mK

Km=0.56

For grids with ground rods in the corners, as well as along the perimeter and throughout

the grid, the effective buried length LM calculated as follows [37].

2 21.55 1.22 r

m c R

x y

LL L L

L L

(3.43)

Where

Lm is effective burial length in meter

LC is the total length of conductor in the horizontal grid in meter.

LR is the total length of all ground rods in meter

Lr is the length of each ground rods in meter

Lx is the maximum length of the grid in the X direction in meter

Ly is the maximum length of the grid in the Y direction in meter

2 2

22000 1.55 1.22 16 2024.83

1000 1000mL m

Now the maximum mesh voltage will be

328.23 0.56 2.124 21150

4077.9562024.83

mE V


75

The safety of a human or an animal depends on preventing the critical amount of shock

energy from being absorbed before the fault is cleared and the system energized. In a

properly design grounding system the maximum mesh voltage must be lower than the

tolerable touch voltage [45].

In IEEE 80 std 2013 two formulas have been driven for calculating the touch voltage by

considering the human average weigh as 50 and 70 Kg. To consider the worst case the

researcher selected 50Kg.

50

0.1161000 1.5touch s s

s

E Ct

(3.44)

70

0.1571000 1.5touch s s

s

E Ct

(3.45)

Where

Etouch50 is the touch voltage in V

Cs is the surface layer derating factor

Ps is the resistivity of the surface material in Ω-m

ts is the duration of shock in sec

6

50

0.116(1000 1.5 0.36 1.5 10 ) 133043.5

0.5touchE x V

Here the calculated mesh voltage is lower than the touch voltage there for the design is

safe.

3.14.8 Step voltage

Step voltage is the potential voltage between the feet of a person standing near an

energized grounded object. The maximum step voltage is assumed to occur over a

distance of 1 m, beginning at and extending outside of the perimeter conductor at the

angle bisecting the most extreme corner of the grid. The usual burial depth is between

0.25 m and 2.5 m [37].


76

The step voltage values are obtained as a product of the geometrical factor Ks; the

corrective factor Ki; the soil resistivity ρ; and the average current per unit of buried length

of grounding system conductor.

s i Gs

s

K K IE

L

(4.45)

Where

Es is Step voltage in V

Ks is the geometrical factor

Ki is the corrective factor

p is soil resistivity in Ω-m

LS is effective buried conductor length in meter

For grids with ground rod the effective buried length Ls calculated as:

0.75 0.85s C RL L L (4.46)

0.75 2000 0.85 16 1513.6sL m

According to IEEE Std 80 for rectangle grid Ks, the geometrical factor calculated as

21 1 1 11 0.5

2

n

sKh D h D

(4.47)

The spacing between parallel conductor D is 11.11m and depth of the grid h which is

0.75m and the n is 10 which is calculated in equation 3.61

10 21 1 1 11 0.5 0.27

2 0.75 11.11 0.75 11.11sK

Ki= 2.124 which is calculated in equation 3.51

Based on the above information the maximum step voltage will be:


77

328.23 0.27 2.124 21150

2630.241513.6

sE V

The next step is calculating the tolerable step voltage. According to IEEE 80 it can be

calculated by considering the average weight of persons as 50 Kg.

0.116

1000 6step s

s

E Ct

(3.46)

6 0.1161000 6 0.36 1.5 10 531682.07

0.5stepE V

Here also the maximum step voltage is lower than the step voltage calculated for 50Kg

person. Therefore the design is safe.


78

3.15 AMIMTDE Substation Layout

As already discussed in busbar arrangment section 3.2, the selected arrangment type is

double busbar double breaker system. It has two main busbars with two circuit breakers

and four switches for each incomming bays and for each feeder circuits

Double busbar double breaker system is more reliabile than any other confiurations. In

this arrangment the two identical busbars have equal current carrying capacity and any

one of them can handle the entire load of the enterprise. This means if one of the busbar

get fulty, the other can carry the all load.

The double busbar arrangment gives a freedom of on load change from one busbar to the

other and any outgoing bay at an any time can be transferred from one busbar to the other

bus. Since the changeover from one bus to the other is done through circuit breakers than

using isolators, it removes the chance of isolators interlocking problem. The selected

arrangmen gives a chance of circuit breaker and isolators maintenance without

interruption of the system. All this makes it higly reliable than any of the arrangment.

One of the disadvanage of using double busbar arrangment is its high cost. As mentioned

in section 3.3 double busbar double breaker system is made from two single busbar

therefor, the enterprise can built single busbar with single breaker system but leave

enogh space for future expansion to double busbar double breaker system.


79

Figure 20 Single line diagram of AMIMTDE substation


80

CHAPTER FOUR

4. SIMULATION RESULT AND DISCUSSION

4.1 Introduction

The simulation part concentrates on the load flow analysis using the Electrical Transient

Analyzer Program (ETAP) which focuses on how the stability of the existing Bahir Dar

substation II is affected because of the introduction of the new proposed AMIMTDE

substation.

The major problem expected during the addition of load on the existing system is steady-

state stability which makes the system to be under voltage. Steady-state stability can be

studied by using load flow analysis which help us to study the system voltage operation

whether it is within the specified voltage limits or not.

As mentioned before, the software which is used to study the load flow analysis is ETAP.

ETAP is an electrical transient analyzer program that is developed under an established

quality assurance program and is used worldwide as high impact software. It is a fully

graphical electrical power system analysis program that runs on Microsoft Windows

operating systems. In addition to the standard offline simulation modules, ETAP can

utilize real-time operating data for monitoring &simulation, optimization, and high-speed

intelligent load shedding. The program is designed with the concept of virtual real

operation, total integration of data and simplicity in data entry.

ETAP load flow analysis performs power flow analysis using the Newton Raphson

method and voltage drop calculations with accurate and reliable results. Built-in features

like automatic equipment evaluation, alerts and warnings summary, load flow result

analyzer, and intelligent graphics make it the most efficient electrical power flow analysis

tool available today [46].

The power flow study concern of this thesis is on evaluating the difference of the system

performance of Bahir Dar substation II before and after connecting the new designed

substation, AMIMTDE substation, so that control measures can be used when necessary.


81

Figure 21 shows single line diagram of Bahir Dar substation II and AMIMTDE

substation. As shown in the figure four generators of Tana Beles each with 133MVA

rating, Tis Abay I with 4.8 MVA and Tis Abay II with 40MVA rating connected.

AMIMTDE substation is connected with Bahir Dar substation with LILO connection

system.


82

Figure 21: Single line diagram of Bahir Dar II and AMIMTDE substations.


83

4.2 Load Flow Analysis

4.2.1 Load flow analysis of Bahir Dar Substation II

The first part of the load flow study is concerned with Bahir Dar substation II only, that means

without connecting the new design AMIMTDE substation and the result of the simulations are

summarized in Table 21.

Figure 22: Load flow simulation of Bahir Dar SS II without AMIMTDE substation.


84

Table 19 Load flow on Bahir Dar SS by disconnecting AMIMTDE SS

Bus Name MW Loading Mvar Loading %PF

400kV BB 457.098 159.514 94.41

230kV BB 272.607 150.339 87.55

132kV BB 39.642 36.451 98.7

66kV BB 87.992 31.206 94.25

33kV BB 1.594 0.656 92.47

Table 20 Load flow analysis summary on busbars operating voltage by disconnecting

AMIMTDE SS

Bus Name Nominal voltage on

BB (kV)

Actual voltage on

BB (kV)

Operating voltage

of Bus (%)

400 kV BB 400 395.703 99.93

230kV BB 230 222.706 96.83

132kV BB 132 131.384 99.53

66kV BB 66 55.225 83.67

33kV BB 33 27.387 82.99

4.2.2 Load flow analysis of Bahir Dar SS by connecting AMIMTDE SS

The next analysis is connecting the newly designed AMIMTDE substation with the

existing Bahir Dar substation. As discussed in chapter two AMIMTDE substations will

have the following feeder line that will connect to distribution transformers.

Table 21 Loads connected to AMIMTDE Substation

ID Type Power (MVA)

Feeder 1 General flexible shop 9

Feeder 2 Farm implement and assembly shop 9

Feeder 3 Electrical machines production shop 9

Feeder 4 Inductive ferrous furnace I 6.3

Feeder 5 Inductive ferrous furnace II 6.3

Feeder 6 Inductive nonferrous furnace I 2

Feeder 7 Inductive nonferrous furnace II 2


85

Figure 23: Load flow simulation of Bahir Dar SS II with AMIMTDE SS connected


86

The result of the load flow analysis when AMIMTDE substation connected with the

existing Bahir Dar substation II with live in live out form is summarized in the table

below.

Table 22 Power and PF on Bahir Dar SS II when AMIMTDE SS connected

Bus Name MW Loading Mvar Loading % PF

400kV BB 475.089 160.287 94.75

230kV BB 286.874 150.378 88.56

132kV BB 39.562 22.69 86.75

66kV BB 87.89 31.17 94.24

33kV BB 1.595 0.657 92.46

Table 23 Load flow analysis summary on busbars operating voltage with AMIMTDE SS

connected.

Bus Name Nominal voltage on

BB (kV)

Actual voltage on

BB (kV)

Operating voltage

of Busbars (%)

400kV BB 400 395.008 98.75

230kV BB 230 222.089 96.56

132kV BB 132 130.389 98.78

66kV BB 66 55.014 83.35

33kV BB 33 27.28 82.67

To study the effect of the AMIMTDE substation on Bahir Dar substation II, the

researcher takes two parameters, the power factor and operating voltage of the busbars

before and after connecting the AMIMTDE substation.


87

Table 24 Difference in Operating voltage and %PF

Bus ID

Operating voltage of busbars (%) Percentage power factor (%PF)

Before After The difference in

operating voltage

Before After The difference in

power factor

400kV BB 99.93 98.75 1.18 94.41 94.75 -0.34

230kV BB 96.83 96.56 0.27 87.55 88.56 -1.01

132kV BB 99.53 98.78 0.75 98.7 86.75 11.95

66kV BB 83.67 83.35 0.32 94.25 94.24 0.01

33kV BB 82.99 82.67 0.32 92.47 92.46 0.01

From table 21 the addition of AMIMTDE substation doesn’t affect the international as

well the Ethiopian standards of voltage drop which is 5% and power factor which is

greater than 85% but the power factor on 132kV busbar is dropped significantly there for

there should be correction or placement of capacitor to compensate the difference

occurred which is 11.95.

Generally, capacitor banks are installed in a substation for improving system capacity,

power factor correction and reactive power control. Capacitor placement involves

determining the capacitor size by calculating the angle difference between the required

and the actual angle.

The power factor of Bahir Dar substation II 132kV busbar before AMIMTDE connected

was 98.7% and after the connection it becomes 86.75% that means the angle theta was

9.25 degrees and after the connection, it becomes 29.83 degrees.

Now the Mvar calculated as follows:-

1 2var ( ) (Tan Tan )M Active power MW

Where θ1 is the existing power factor

θ2 is the required power factor

var 39.562 (tan 29.83 tan9.25)M

var 39.562 0.411 16.24 varM M


88

During simulation, placing single 16.24Mvar on 132 kV busbar of Bahir Dar substation II

cannot bring up the power factor as required but placing it on AMIMTDE 132kV busbar

makes the power factor to rise to 96.5%.


89

CHAPTER FIVE

5. CONCLUSION AND RECOMMENDATIONS

Based on the thesis work which studies on substation design and modeling for Amhara

Metal Industry and Machine Technology Development Enterprise with IEEE standards,

this section discusses the conclusions, important recommendations and the suggested

areas for future research work.

5.1 Conclusion

The base for a reliable substation starts from design. Up to now our country doesn’t have

guides or standards to design a substation; therefore, we need to follow internationally

accepted standards and guides.

IEEE has 50 standards, guides and recommended practices for substation design,

construction and operation. The thesis bases on the listed standards as a guideline.

IEEE standard 605 ™-2008 guides for bus design in air-insulated substations.

IEEE standardC57.12.00TM

-2015 general requirements for liquid immersed

transformers

IEEE standard81™-2012, a guide for measuring earth resistivity, ground

impedance and earth surface potentials of a ground system.

IEEE standard80™-2000, a guide for safety in AC substation grounding.

IEEE standard998™-2012,a guide for direct lightning stroke shielding of

substations.

These are the major ones otherwise every design, material selection of the thesis bases on

IEEE standards.

The thesis has covered all the necessary methods and calculations to design a substation.

This includes load forecasting, connection options to connect the new substation with the

existing Bahir Dar substation II, busbar arrangements, rating of major substation

equipment, earth mat design and lightning stroke shielding.


90

After completing the design work, the thesis focus on the new designed substation effect

on the existing Bahir Dar substation II by using ETAP software. The effect study focus

on voltage stability and power factor variation before and after the new substation

connects to the existing system.

As the simulation shows the new substation doesn’t affect the voltage stability but it

decreases power factor of 132kV busbar but using capacitor bank it is compensated.

5.2 Recommendation

Following the increase in power demand in our country, there will be a lot of requests for

new substations and upgrading the exiting substations. But emphasis needs to be given on

the reliability of the system which starts from design. Since the international accepted

IEEE has standards, guides and recommended practices for each individual designs and

construction works of a substation therefore by base on it Ethiopia Electric Agency needs

to have its own standards and guidelines.

5.3 Future Work

The following tasks are suggested as future work.

1. The thesis focuses on the steady-state analysis but it needs to study transient

analysis and more work should be done to improve the power quality by

considering disturbance.

2. GIS and air-insulated substations have different in construction, installation and

cost, therefore study needs to be conducted on GIS especially for customer

substations because it may be more feasible in terms of cost and management.

3. For increasing reliability of the substation, more study needs to conduct on

automation systems.


91

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Appendix

Appendix 1. Ambacity table for aluminum tubular busbar

SPS

size

(in)

OD

(in)

Wall

thickness

(in)

Emissivity equals to 0.5, with sun

Temperature rise above 40 oC ambient

30 40 50 60 70 90 110

1.0 1.315 0.133 572 690 788 872 948 1078 1190

1.5 1.9 0.145 805 981 1127 1252 1363 1556 1723

2.0 2.375 0.154 991 1217 1402 1561 1703 1949 2161

2.5 2.875 0.203 1314 1623 1876 2094 2287 2623 2914

3.0 3.5 0.216 1582 1969 2284 2555 2795 3214 3576

3.5 4.0 0.226 1796 2248 2614 2929 3208 3694 4116

4.0 4.5 0.237 2015 2534 2954 3315 3635 4192 4675

5.0 5.563 0.258 2474 3142 3680 4141 4550 5262 5880

6.0 6.625 0.28 2943 3771 4435 5003 5506 6382 7144

8 8.625 0.322 3830 4982 5899 6681 7373 8581 9633

Appendix 2. Fault current level and withstand time for aluminum and copper

Material Fault current

level (kA)

Withstand time

1 sec 200ms 40ms 10ms

Aluminum

35 443 198 89 44

50 633 283 127 63

65 823 368 165 82

Copper

35 285 127 57 28

50 407 182 81 41

65 528 236 106 53

Appendix 3. Percent impedance at self -cooled (ONAN) rating

High-Voltage BIL

(kV)

Percentage

impedance

110 or less 5.5

150 7

200 7.5

250 8

350 8.5

450 9

550 9.5

650 10

750 10.5


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Appendix 3. Tis Abay I, Tis Abay II and Tana Beles generators data

Generators voltage

(kV)

MVA Inertia

H(s)

T’do T”do Xd Xq X’d X”d

Tis Abay1 6 4.8 2 5.4 0.05 1.21 0.7 0.3 0.18

Tis Abay2 10.5 40 2.5 7.1 0.12 1 0.6 0.29 0.36

5

Tana Beles 15 133 3.14 9.2 0.1 1.03 0.7 0.31 0.25

Appendix 4. Transmission lines parameters that are connected to Bahir Dar substation II

Starting Bus Destination

Bus

Length

(KM)

MVA R(pu) X(pu) B(pu)

Bahir Dar II Alamata 341 318 0.020703 0.063055 0.1906

Bahir Dar II Motta 83 280 0.012784 0.0664290 0.1200

Bahir Dar II Gondar 137 318 0.029318 0.0844430 0.2552

Bahir Dar II D/Markos 193.7 1341 0.002095 0.0285847 0.7905

Bahir Dar II Tana Beles 62.84 1341 0.000679 0.0087208 0.2704

Bahir Dar II Dangila 68.6 24 0.012784 0.0637841 0.0127

Appendix 5. Transformers Parameters

Voltage (kV) Rating

(MVA)

R (%) X (%) X/R ratio

400/230 133 0.176 12.045 68.44

400/15 133 0.215 13.5 62.79

230/132 133 0.233 13.36 57.33

230/66 133 0.43 12.368 28.76

230/33 6.3 0.302 8.4 27.78

132/10.5 24 0.257 7.8 30.35

132/6 6.3 0.3221 6.387 28.85


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Appendix 6. Details on major substation components

S/N Components

Data

Quantity Description

Value

1 Tana Beles

Generators 4

MVA 133

kV 15

%PF 90

Efficiency 98

Pole 16

Qmax(MVAR) 127.6

Qmin(MVAR) -87.8

Impedance % (Ω)

Xd’’, X2, X0

0.17, 0.25, 0.12

Armature resistance

Ra, R2, Ro

0.01,0.02,0.025

2 Tis Abay I 1

Rating (MVA) 4.8

kV 6

%PF 83

Efficiency 95

Pole 16

Qmax 2.677

Qmin 0

Impedance % (Ω)

Xd’’, X2, X0

0.15, 0.22, 0.08

Armature resistance

Ra, R2, Ro

0.021,0.02,0.005

3 Tis Abay II 1

MVA 40

kV 10.5

%PF 98

Efficiency 98%


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Pole 28

Qmax 7.695

Qmin 0

Impedance % (Ω)

Xd’’, X2, X0

0.245, 0.212,

0.092

Armature resistance

Ra, R2, Ro

0.018, 0.02,0.005

4 Shunt reactor 1

Rated voltage 400

MVA 50

Reactor PF 5

5 Transformer 1 1

Primary voltage 15

Secondary voltage 400

Rated MVA 133

% Z 13.5

X/R 63

5 Transformer 2 1

Primary voltage 400


Rated MVA 133

% Z 14.02/10.01/8

X/R 20/10/200

6 Transformer 3 2

Primary voltage 230


Tertiary voltage 15

Rated MVA 63/21/21

% Z 9.3/8.3/4.7

X/R 23.846/19.76/10

7 Transformer 4 1

Primary voltage 230


Tertiary voltage 15

Rated MVA 63/40/23


100

% Z 14,10,8

X/R 20/10/200

8 Transformer 5 2

Primary voltage 132


Rated MVA 50

% Z 12

X/R 40

9 Transformer 6 1

Primary voltage (kV) 10.5


Rated MVA 50

% Z 7.804

X/R 30.35

10 Transformer 7 1

Primary voltage (kV) 6


Rated MVA 6.3

% Z 6.39

X/R 28.85

11 Transformer 8 1

Primary voltage (kV) 66


Rated MVA 12

% Z 8

X/R 17

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