International Journal of Electrical Engineering & Technology (IJEET)
Volume 7, Issue 5, September–October, 2016, pp.32–43, Article ID: IJEET_07_05_004
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SRF THEORY BASED STATCOM FOR
COMPENSATION OF REACTIVE POWER AND
HARMONICS
Satyanarayana Gorantla
Associate Professor in EEE Department, Anurag Engineering Collage,
Kodad & Research Scholar in A.N.U, Andhra Pradesh, India.
Gili Ravi Kumar
Profissor & HOD of EEE Department, Bapatla Engineering College,
Andhra Pradesh, India.
ABSTRACT
The power electronic devices like converters and inverters inject harmonic currents into AC
system due to their non linear characteristics. These devices draw high amount of reactive power
from source. The commencement of Nonlinear Load into the ac power system will have the effect of
harmonics. The presence of harmonics in system it will effected with power quality problems. Due
to this high amount of power losses and disoperation of power electronics devices is caused, along
with this Harmonics have a number of undesirable effects like Voltage disturbances. These
harmonics are needed to mitigate for Power Quality Enhancement in distributed system. Here the
device called STATCOM is one of the FACTS Devices which can be used to mitigate the harmonics
and reactive power compensation. The voltage source converter is core of the STATCOM and the
hysteresis current control is indirect method of controlling of VSC. In this paper we implement with
SRF based STATCOM control. SRF theory is implemented for the generation of controlling
reference current signals for controller of STATCOM. The Matlab\Simulink based model is
developed and simulation results are showed for linear and nonlinear load conditions.
Key words: Harmonics compensation, Power quality, Reactive power, STATCOM, Synchronous
reference frame theory (SRF).
Cite this Article: Satyanarayana Gorantla and Gili Ravi Kumar, SRF Theory Based STATCOM
for Compensation of Reactive Power and Harmonics. International Journal of Electrical
Engineering & Technology, 7(5), 2016, pp. 32–43.
http://www.iaeme.com/IJEET/issues.asp?JType=IJEET&VType=7&IType=5
1. INTRODUCTION
In Modern distribution systems have very complex networks connected with linear and Non linear loads.
Generation of electricity without any power quality is very complex, so in power system faces various
types of power quality problems which vary in their scope and severity. While delivering power to the
consumers, power quality is the most important criteria to look for. Power quality is how far the practical
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SRF Theory Based STATCOM for Compensation of Reactive Power and Harmonics
system resembles the ideal conditions. Power quality is defined as providing uninterrupted power supply
with voltage, frequency and power factor within nominal values [1].
Non linear loads are primary thing for harmonic distortion in a power system these harmonic
distortions will be eliminated so far using tuned LC filters\HPF and shunt passive filter. In shunt passive
filter have an resonance and fixed compensation problems, the disadvantage of passive filters is overcome
by using FACTS controller STATCOM.
The Concept of STATCOM is proposed by Gyugyi in 1976. A STATCOM is operated as a shunt
connected static var compensator (SVC) whose capacitive or inductive output current can be controlled
independent of the ac system voltage. STATCOM is used to improve power quality and compensate
harmonics of the system by injecting compensating currents. The performance of an STATCOM mainly
based on the reference current generation strategy, current control technique, power inverter topology
which is done by controller designed for STATCOM. One of the important tasks in the STATCOM design
is the maintenance of constant DC voltage across the capacitor connected to the inverter [5].In this
investigation, STATCOM has been realized using 3-φ 3-leg VSC, which is controlled for reactive power
compensation and harmonic elimination [6].
The STATCOM adopt intelligent control circuits to detect harmonic and reactive power drawn by
nonlinear loads and take corrective actions to make the source current purely sinusoidal at source.
Synchronous reference frame theory (SRF) based algorithm is found to have a better performance than
other control algorithms for STATCOM [7].
In this paper we tend to implement the STATCOM compensating the reactive power and harmonics in
distribution system. The Matlab\Simulink based model is developed and simulation results are showed for
linear and nonlinear load conditions.
2. STATCOM CONNECTED IN DISTRIBUTION SYSTEM
Figure 1 Block diagram for STATCOM connected in distribution system.
The STATCOM basically consists of a VSC means voltage source converter with a leakage reactance. A
capacitor in its DC side connected the inverter is operated as in conventional STATCOM mode, switched
with a SPWM and in order to provide harmonic current minimization and serve as a link between VSI and
the system [8]. The block diagram of STATCOM connected in Distribution system is showed in figure.1.
The DC voltage to VSI is maintained with a capacitor is connected across STATCOM. When non
linear connected with the system the load and STATCOM currents are unbalanced in nature. The AC
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Satyanarayana Gorantla and Gili Ravi Kumar
voltage difference of system, across the leakage reactance produces reactive power exchange between the
STATCOM and the power system, such that the AC voltage at the bus bar can be regulated to improve the
voltage profile of the power system, which is the primary duty of the STATCOM
If the system voltage is more than the voltage source inverter voltage, then the STATCOM absorbs the
reactive power from source. Similarly if the system voltage is less than the voltage source inverter voltage,
then the STATCOM generates reactive power to load. Other than these two cases the system voltage is
equal to the voltage source inverter voltage [5].
3. CONTROL TECHNIQUE FOR STATCOM
3.1. Synchronous Reference Frame Theory (SRF) Theory
Figure 2 Block diagram for SRF theory based control.
The internal view of Synchronous Reference Frame Theory (SRF) control strategy for STATCOM is
showed in figure.2. In this control strategy the SRF-based STATCOM control technique is used to
generate gate pulses for controlling of STATCOM. Here from the control strategy is designed with abc
frame to d-q frame conversion block, PLL block, HPF, PI controller, DQ to ABC conversion block and
hysteresis controller [10]-[11].
In abc frame to d-q frame conversion block is used to convert three phase load current parameters
(Iabc) to dq0 parameters frame by using parks transformation. Firstly, the current components from block
in α–β co-ordinates are generated. Iabc phases can then be transformed into α– β coordinates. The phase
locked loop is used to generate Sinwt and coswt signals for transformation block. HPF is high pass filter is
used to block low frequency components coming from conversion block. The Vd reference and actual
value is compared then given to pi controller is again compared with Id loss value then given to inverse
transformation block. The output of inverse transformation block is STATCOM reference current is
compared with actual STATCOM current then given to Hysteresis controller. It will generate gate pulses
for switches.
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SRF Theory Based STATCOM for Compensation of Reactive Power and Harmonics
4. COMPLETE BLOCK DIAGRAM OF SRF CONTROLLED STATCOM
T1
T
T
4
T
3
T
6
T
5
2
Figure 3 Block diagram for SRF theory based control of STATCOM connected in Distribution system.
The Block diagram for SRF theory based control of STATCOM connected in Distribution system is
showed in figure.3. The control strategy for STATCOM is consists voltage source inverter (VSI), SRF
theory control technique and distribution, interfacing inductances and phase locked loop (PLL).
The performance of STATCOM as a compensator depends on the control algorithm what we are used
here i.e. the precise extraction of the current with harmonics reactive components. For this purpose there
are so many control schemes which have been proposed and are reported in the literature and three of these
are the Instantaneous Reactive Power Theory (IRPT), Synchronous Reference Frame Theory (SRF) and Icosф algorithm which are most widely used. In this paper, controlling of STATCOM by SRF algorithm for
compensation of reactive power and harmonics component has been used. Due to non linear connected
loads in system will demands for reactive power from source, so for this purpose of compensating reactive
power and harmonics at PCC the STATCOM is shunted to line through interfacing instances [10].
5. ESTIMATION OF REFERENCE REACTIVE SOURCE CURRENT
The instantaneous currents can be written as
is (t ) = il (t ) − ic (t )
(1)
Source voltage is given by
vs (t ) = vm sin ω t
(2)
If a non-linear load is applied, then the load current will have a fundamental component and harmonic
components which can be represented as
∞
∞
iL (t ) = ∑ I n sin(nωt + φn ) = I l sin(nωt + φl ) + ∑ I n sin(nωt + φn )
n =1
n=2
(3)
The instantaneous load power can be given as
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Satyanarayana Gorantla and Gili Ravi Kumar
pL (t) = vs (t) * il (t)
= vmil sin2 ωt *cosφ1 + vmil sin ωt *cos ωt *sin φ1
∞
+vm sin ωt * ∑ In sin(nωt + φn )
(4)
n =2
= Pfund (t ) + Preac (t ) + Phar (t )
(5)
the real (fundamental) power drawn by the load is
Pfund (t ) = vm il sin 2 ω t * cos φ1 = vs (t ) * is (t )
(6)
the source current supplied by the source, after compensation is
is (t ) =
Pfund (t )
vs (t )
= il cos φ1 sin ωt = I m sin ωt
(7)
The desired source currents, after compensation, can be given as
isa* (t ) = I sp sin ω t
isb* (t ) = I sp sin(ω t − 120 o )
isc* (t ) = I sp sin(ω t + 120o )
(8)
Where Isp (=I1cosΦ1+Isl) the amplitude of the desired source current of system,
The load requires real power from ac source is supplied with STATCOM configuration and few losses
like switching losses, losses in reactor and dielectric losses. The STATCOM switching is done by
reference source current is used here it has two elements those are real first harmonic component and
losses in STATCOM. The first harmonic component of load current being extracted from SRF theory. The
losses of STATCOM is estimated mistreatment a proportional–integral (PI) controller over the dc bus
voltage of the STATCOM. To compute the second element of the reference active current, a reference dc
bus voltage (Vdc*) is compared with the sensed dc bus voltage (Vdc) of STATCOM. Which in the nth
sampling instant, is expressed as
*
vdcl ( n) = vdc
( n) − vdc (n)
(9)
th
This error signal vdcl(n) is processed in a PI controller, and the output {Ip(n)}at the n sampling instant
is expressed
Ip(n) = Ip(n −1) + K pdc{vdcl (n) − vdcl (n−1) } + Kidcvdcl (n)
(10)
Where Kpdc and Kidc are the proportional and integral gains of the PI controller. The output of this PI
controller accounts for the losses in STATCOM, and it is considered because of the loss element.
6. MODELLING AND SIMULATION
Simulations studies were carried out to study the behavior of STATCOM under different loads like linear
and non linear loads. The simulating parameters values are showed in table.1
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SRF Theory Based STATCOM for Compensation of Reactive Power and Harmonics
Table 1 Simulation Parameters
PARAMETER
VALUE
Source voltage
415V
Source Imp
0.1+j0.027
20+j0.002
Load Imp
Inter facing inductance Linear load (RL+LL)
Inter facing inductance at Non linear load
(RL+LL)
0.1+j0.001
0.1+j0.003
6.1. Performance of STATCOM for Linear Load Condition
Figure 4 Source Voltage waveform at Linear Load
Figure 5 Source current waveform at Linear Load
Figure.4 & Figure.5 shows the Source voltage and current waveforms at linear load connected in
Distribution system. The peak amplitude of source voltage is 320V and the peak amplitude of source
current is 15A.
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Satyanarayana Gorantla and Gili Ravi Kumar
Figure 6 Load current waveform at Linear Load
Figure.6 shows load current wave form linear load. Here R-L load is considered, it has peak amplitude
of load current is 15A.
Figure 7 STATCOM current waveform at Linear Load
Figure.7 shows the compensating STATCOM current waveforms at linear load connected in
Distribution system. The STATCOM supplies the Reactive current required by the load.
Figure 8 Power Factor waveform at Linear Load
Figure.8 shows the Power Factor waveforms at linear load connected in Distribution system. Here the
power factor is unity , because STATCOM compensate the reactive current required by the load.
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SRF Theory Based STATCOM for Compensation of Reactive Power and Harmonics
6.2. Performance of STATCOM with Non-Linear Load Condition
The performance of STACOM with SRF controlled non-linear load can also be observed and results has
been showed in below with clear explanation.
Figure 9 Source voltage waveform at Non Linear Load
Figure10 Source current waveform at Non Linear Load
Figure.9 & Figure10 shows the Source voltage and current waveforms at nonlinear load connected in
Distribution system, The peak amplitude of source voltage is 320V and the peak amplitude of source
current is 30A.
Figure 11 load current waveform at Non Linear Load
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Satyanarayana Gorantla and Gili Ravi Kumar
Figure.11 shows the load current waveforms at non linear load connected in Distribution system. Due
to non linear load nature the current shape is square wave and it contains harmonics is showed.
Figure 12 STATCOM current waveform at Non Linear Load
Figure.12 shows the compensating STATCOM current waveforms at nonlinear load connected in
Distribution system. STATCOM supplies the harmonic current to load, such that source current is
sinusoidal.
Figure 13 Load current THD waveform at Non Linear Load
Figure.13 shows the load current THD plot at Non linear load connected in Distribution system. The
value of THD presented in load current is 27.48%.
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SRF Theory Based STATCOM for Compensation of Reactive Power and Harmonics
Figure 14 Source current THD waveform at Non Linear Load
Figure.12 shows the Source current THD plot when Non linear load is connected in Distribution
system. The THD at source current is 1.52%.
Figure 15 Power factor waveform at Non Linear Load
Figure.15 shows the Power Factor waveforms at non linear load connected in Distribution system.
Here the power factor is unity , because STATCOM compensate the reactive current required by the load..
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Satyanarayana Gorantla and Gili Ravi Kumar
Table 2 THD Values
PARAMETERS
THD of Source
current without
STATCOM
Nonlinear load
23.03%
THD of Source
current with
STATCOM
1.52%
7. CONCLUSION
The Matlab\Simulink based model is simulated and results are discussed. The performance analysis of
proposed STATCOM with SRF control theory is found to be quiet satisfactory for harmonic elimination
and reactive power compensation for linear and non linear loads. The STATCOM effectively compensate
the reactive power & harmonics for linear and non linear load cases. For non linear load case the effect is
compensated by STATCOM and power factor is maintained at unity. The harmonic content of the current
at source side is 1.52% when non linear load is connected.
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