A DSTATCOM for Enhancement of Power Quality in Distribution Systems

1Lakshman Naik Popavath, 2K.Palanisamy

School of Electrical Engineering,

VIT- University, Vellore, India.

Abstract

At present the most important power Quality issues in distribution and transmission systems faced by both customers and utilities are poor Power Factor and voltage variations (Voltage Sag, Swell). The

voltage sags will effect on the performance of the sensitive equipments like Robotics, Programable

Logic Controllers, ASD, high intensity discharge Lamps etc. And also on the nation Economy. The

voltage sags will be mitigated by applying various conventional methods. This paper elaborates the

design approach and the dynamic performance of 3-level Voltage Source Converter based D- Statcom for PF improvement and for the mitigation of voltage sags due to various faults in an electric

distribution system.The simulation work has been carried out in MATLAB (2009b) platform to mitigate

the voltage sags due to SLG,DLG,LL,TLG faults in the proposed system. In this research work the

PF has enhanced from non-unity PF stage to Unity PF stage.

Index Terms: Distribution system, Faults, D-Statcom, VSC, Voltage Sag mitigation, PF

Improvement.

1. INTRODUCTION

In recent years an increased utilization of non-linear loads causing power quality concerns

(power pollution) in distribution and transmission systems. The polluted power will effect on the performance and life span of the system in most cases PQ means the quality of voltage that is being

address in the system. Power quality concerns faced by both the customers and utilities are poor

power factor and voltage variations (voltage sag and swell). By the definition the voltage sag is

defined as the reduction in RMS voltage (AC voltage) from 0.9pu to 0.1pu value at power frequencies

within 0.5 cycles to 1min [8]. Voltage sag contributes 80% plus power quality problems which exist in

distribution and transmission systems [4]. The voltage sag is mainly characterized by two factors magnitude/depth and duration [7]. The main important sources of voltage sag are large increase of

load current [like transformer energization or starting of large motors], occurrence of faults in the

utility system, faults within the customers facility [7]. The voltage sags will effect on the active

performance of sensitive equipment like high intensity discharge lamps, PLG, ASD, and robotics etc.

Various conventional methods (capacitor bank parallel feeder, UPS) are applied to mitigate

the voltage sags, but these methods are not able to solve the PQ problems completely. Among the various custom power devices the D-STATCOM is a promising device to mitigate power quality

problems like voltage sag, swell, current harmonics, and reactive power control in the distribution and

transmission system. The electronic values of D-STATCOM were controlled by implementing the new

PWM-based control scheme in the proposed work.

In this paper the dynamic performance and designing of D-STATCOM for mitigation of

voltage sag due to faults and enhancement of PF are analyzed for 11kv distribution system. The

simulation work has been carried out in MATLAB [2009b] platform to mitigate the sags due to various

International Journal of Pure and Applied MathematicsVolume 119 No. 12 2018, 363-375ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

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faults in the proposed distribution system. The complete organization of the paper is described by the

sections II, III, IV, and V respectivsely as follows.

2. TEST MODEL FOR VOLTAGE SAG MITIGATION

R

D-STATCOM Faults

Fig1- Proposed test model for power Quality

This section mainly describes the configuration and dynamic operation of D-STATCOM for

proposed research work and design of D-STATCOM for voltage sag mitigation. The new PWM-base

D-STATCOM is connected in shunt with the distribution system, as shown in fig. 1.

The test model consists 11kv, 50Hz distribution system with shunt connected DSTATCOM for

voltage sag mitigation.

A. Voltage sag in the proposed system 338

Electrically controlled sensitive equipment like PLC, ASD, Robotics and etc. requires pure supply voltage without any ripple or disturbance [6]. As per the IEEE std. 1159(1995) the reduction in

RMS voltage from 90% to 10% in the duration of half cycle to 1min is termed as voltage sag. There

exist several causes for voltage sag, some of them are switching of CB’s large increase of load

currents (starting of motor, transformer energizing), bad weather conditions, pollution, various faults

within the utilities. The voltage sags are mainly characterized by magnitude/depth and duration. According to IEEE std. the voltage sags are characterized as shown in table 1.

Table-1: Types of Voltage sags

Sag type Magnitude Time duration

Instantaneous 0.9p.u to 0.1p.u 0.5-30 cycles

Momentary 0.9p.u to 0.1p.u 30cycles-3sec

Temporary 0.9p.u to 0.1p.u 3sec-1min

In the proposed work the sags are created due to short circuit faults.

Vs PCC

VL

Xs L1 L2 L3

VPCC

Main Source PSF+jQSF

Linear Load

a b c

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B. Design of STATCOM for sag mitigation

Among all the sag mitigation techniques the D-STATCOM is a promising FACTS device for voltage mitigation. The shunt connected D-STATCOM can generate or absorbs the reactive power.

The new PWM statcom can generate the voltage with required magnitude, frequency and phase

angle [7]. The missing voltage can be injected by Statcom to nullify the voltage sags in the test

system. The difference between the nominal voltage and actual voltage is termed as sag voltage (or)

missing voltage. The design of statcom for voltage sag mitigation is carried out as follows.

B.1. Capacitor sizing

Suitable capacitor sizing is very important to mitigate the voltage sag [4] due to faults in the

proposed system. The DC capacitor CDC will play a vital role to inject the reactive power to D-

STATCOM when the test system is under the sag condition [4]. The capacitor designing will be described by the following equation.

1 C [V

2 V

2 ]

1 (V I

.T )

2 dc C max dc

2 s L

The equation for 3-phase system is given as 339

C 3 * Vs I L .T

dc [V 2 V 2 ]

Where

Vs =Peak phase voltage

IL =Drop in load current

T =Time duration of voltage and current

Vdc =Voltage across the Capacitor

VC max =Pre-set upper limit of energy storage in C

C max dc

Vdc

Where "m" is modulation index and it is treated as '1'.

3. PWM controller for DSTATCOM

The controller system is partially a part of the proposed system. Because of its low switching

losses the PWM control strategy is applied to control the electronic values of voltage source converter (VSC) to generate the gate signals to the D-STATCOM to mitigate the voltage sag in the

proposed research work. This is the suitable techniques to regulate the phase angle of the injected

voltage [6]. The PI controller will balance the actual or true voltage (RMS voltage) and reference

voltage. The complete control algorithm will be explained with help of the fig.2 as shown below.

2 2.VLL

3.m

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Fig2- Complete control scheme for proposed system.

The active current component is calculated with help of the following equation for PI controller.

I m(k )

I m(k 1)

K PV

dc(k ) (V

err(k ) V

err(k 1) )

K I V

dc(k )V

err(k )

The dc-link error voltage can be obtained as

Verr (k )

Vdcref (k )

Vdc(k )

Where “k” is sampling instant and Verr is dc-link error voltage. In the control block the PWM generator

generates the sinusoidal PWM waves or signals for D-STATCOM operation.

4. RESULT ANALYSIS The MATLAB/Simulink result shows the dynamic performance of DSTATCOM for voltage sag

mitigation and for PF enhancement in distribution system. The various faults such as single line to

ground (SLG), Line to Line (LL), Double line to ground (DLG) and Triple phase to ground (TPG) faults

are created in the test model. The MATLAB results are analyzed for with and without Stacom.

A. Without DSTATCOM

The faults are created from 0.1 sec to 0.2 sec in the test model. The voltages reduced for Fault

resistance Ron=0.86 Ω from 1p.u Line voltage due to various faults such as SLG,LL,DLG,TPG faults

as shown in from Fig.3 to Fig.6. Voltage Sag

Fault D

uration

SLG Fault

Time(sec)

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Fig3- Sag Voltage due to SLG Fault

Voltage Sag

Fault

Duration

LL Fault

Time (sec)

Fig4- Sag Voltage due to LL Fault.

Voltage Sag

LLG Fault

Fault D

uration

Time (sec)

Fig5- Sag Voltage due to LLG Fault.

Sag Voltage

TPG Fault

Fault D

uration

Time (sec)

Fig6- Sag Voltage due to TPG Fault.

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Table-2: Voltage sags without DStatcom

Fault

Resistance

Ron Ω

Without DSTATCOM

Sag Voltages with different Faults

in (p.u)

SLG Fault

LL Fault

DLG Fault

TPG Fault

Ron=0.65 0.824 0.755 0.701 0.655

Ron=0.76 0.849 0.792 0.745 0.712

Ron=0.86 0.868 0.821 0.784 0.752

Ron=0.96 0.887 0.850 0.818 0.793

Ron=0.99 0.893 0.859 0.829 0.805

The table.2 indicates overall sag voltages due to various faults. From the table it is clearly observed

that at fault resistance Ron=0.86 Ω the voltages are reduced from 1p.u to 0.868p.u,0.821p.u,0.784

p.u,0.752p.u for SLG,LL,DLG,TPG faults respectively.

B. With DSTATCOM

With the insertion of DStatcom the sag voltages are mitigated for various faults as shown in from

Fig.7 to Fig.10. Sag Voltage with DSTATCOM

Fault Du ration

SLG Fault

Time (sec)

Fig7-Improved Sag Voltage of SLG fault with DStatccom

Sag Voltage with DSTATCOM

Fault D

uration

LL Fault

Time (sec)

Fig8- Improved Sag Voltage for LL fault with DStatccom

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Sag Voltage with DSTATCOM

Fault D

uration

LL Fault

Time (sec)

Fig9-Improved Sag Voltage of LLG fault with DStatccom

Sag Voltage with DSTATCOM

Sag Duration

TPG Fault

Time (sec)

Fig10-Improved Sag Voltage for TPG with DStatccom.

Table-3: Improved Voltage sags with DStatcom

Fault Resistance

Ron Ω

With DSTATCOM

Sag Voltages at different Faults in (p.u)

SLG Fault

LL Fault

DLG Fault

TPG Fault

Ron=0.65 0.984 1.017 0.980 0.936

Ron=0.76 0.982 1.014 0.981 0.945

Ron=0.86 0.986 1.015 0.985 0.954

Ron=0.96 0.991 1.016 0.991 0.965

Ron=0.99 0.993 1.016 0.993 0.967

The table.3 indicates the overall improvement in sag voltages for various faults. The sag voltages

from 0.868pu,0.821pu,0.784pu,0.752pu are improved to 0.986p.u,1.015p.u,0.985pu,0.954p.u for

various faults SLG,LL,DLG,TPG respectively.

The Table.4 shows the percentage of voltage sags due to various faults for both without and

with DSTATCOM insertion.

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Table-4: % of Voltage sags with/without DStatcom

Resist ance

Ron

in Ω

% of Sag Voltages at different Faults in (p.u)

Wit hout D-Statcom With-Statcom

SL G

LL DL G

TP G

S L G

L L

D L G

T P G

Ron=0. 17. 22. 29. 34. 1. 1. 2. 6. 65 6 5 9 5 6 7 0 4

Ron=0. 15. 20. 25. 28. 1. 1. 1. 5. 76 1 8 5 8 8 4 9 5

Ron=0. 13. 17. 21. 24. 1. 1. 1. 4. 86 2 9 6 8 4 5 5 6

Ron=0.

96

11. 3

15. 0

18. 2

20. 7

0. 9

1. 6

0. 9

3. 5

Ron=0. 10. 14. 17. 19. 0. 1. 0. 3. 99 7 1 1 5 7 6 7 3

%Sag presagvoltage sagvoltage

*100

presagvoltage

%Sag 1 0.752

*100 1

=24.8% for TPG Fault without D-Statcom.

Table.5 indicates the percentage of mitigated voltage sags with the insertion of D-Statcom.

Table5:% of Voltage sags Mitigation with DStatcom

Fault Resistance

Ron Ω

% of Sag Voltage Mitigation With D-STACOM at different Faults in (p.u)

SLG LL DLG TPG

Ron=0.65 16.0% 20.8% 27.9% 28.1%

Ron=0.76 13.3% 19.4% 23.6% 23.3%

Ron=0.86 11.8% 16.4% 20.1% 20.2%

Ron=0.96 10.4% 13.4% 17.3% 17.2%

Ron=0.99 10.0% 12.5% 16.4% 16.2%

The Fig.11 and Fig.12 depicts the enhancement of PF from non-unity PF state to Unity PF state with the insertion of DSTATCOM.

From Fig.11 it is clear that in fault duration (0.1sec to 0.2 sec) the voltage and current do not have

the desired phase relations, whereas for Fig.12 these are related with desired phase relationship

which indicates the Unity Power Factor condition.

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PF without DSTATCOM

Time (sec)

Fig11-Improper phase relation between V and I due to faults.

PF with DSTATCOM

Sag Dura

tion

Time (sec)

Fig11-Improved phase relation between V and I with DStatcom.

5. CONCLUSION

The Voltage source converter based shunt connected DSTATCOM successfully mitigated the

voltage sag (PQ issue) with help of the new Pulse Width Modulation control technique. The complete controlling action of DSTATCOM is done by employing the PI controller in the control part. The

MATLAB/Simulation results clearly indicating that the dynamic performance of shunt connected

distributed static compensator for mitigation of sag voltages due to various faults and the

enhancement of PF from non-unity state to Unity PF state. The DSTATCOM can also reduce the

harmonics in the distribution and transmission systems.

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