a dstatcom for enhancement of power quality in
TRANSCRIPT
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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