srf theory based statcom for compensation of reactive power and harmonics

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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.

Dr. Goli Ravi Kumar

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 Dr. Goli 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

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

Satyanarayana Gorantla and Dr. Goli 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.



4. COMPLETE BLOCK DIAGRAM OF SRF CONTROLLED STATCOM

1T3T

4T2T6

T

5T

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 I-

cosф 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

( ) ( ) ( )s l c

i t i t i t= − (1)

Source voltage is given by

( ) sin ts m

v t v ω= (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

1

( ) sin( )L n n

n

i t I n tω φ∞

=

= +∑ 2

sin( ) sin( )l l n n

n

I n t I n tω φ ω φ∞

=

= + + +∑ (3)

The instantaneous load power can be given as



L s(t) (t)* (t)l

p v i=

2

1 1

2

sin *cos sin *cos *sin

sin * sin( )

m l m l

m n n

n

v i t v i t t

v t I n t

ω φ ω ω φ

ω ω φ∞

=

= +

+ +∑ (4)

( ) ( ) ( )fund reac harP t P t P t= + + (5)

the real (fundamental) power drawn by the load is

2

1( ) sin *cosfund m lP t v i tω φ= ( )* ( )s s

v t i t= (6)

the source current supplied by the source, after compensation is

1

( )( ) cos sin sin

( )

fund

s l m

s

P ti t i t I t

v tφ ω ω= = =

(7)

The desired source currents, after compensation, can be given as

* ( ) sinsa spi t I tω=

* ( ) sin( 120 )o

sb spi t I tω= −

* ( ) sin( 120 )o

sc spi t I tω= + (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

*( ) ( ) ( )dcl dc dcv n v n v n= − (9)

This error signal vdcl(n) is processed in a PI controller, and the output {Ip(n)}at the nth

sampling instant

is expressed

p(n) p(n 1) ( ) ( 1) ( )I I { }

pdc dcl n dcl n idc dcl nK v v K v

− −= + − +

(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



Table 1 Simulation Parameters

PARAMETER

VALUE

Source voltage 415V

Source Imp 0.1+j0.027

Load Imp 20+j0.002

Inter facing inductance Linear load (RL+LL)

0.1+j0.001

Inter facing inductance at Non linear load

(RL+LL) 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.



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.



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



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%.



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..



Table 2 THD Values

PARAMETERS

THD of Source

current without

STATCOM

THD of Source

current with

STATCOM

Nonlinear load 23.03% 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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