A comprehensive comparison of STATCOM versus SVC-based

Electr EngDOI 10.1007/s00202-017-0559-6

ORIGINAL PAPER

A comprehensive comparison of STATCOM versus SVC-basedfuzzy controller for stability improvement of wind farm connectedto multi-machine power system

M. G. Hemeida1 · Hegazy Rezk1,2 · Mohamed M. Hamada1

Received: 28 October 2016 / Accepted: 5 May 2017© Springer-Verlag Berlin Heidelberg 2017

Abstract Stabilization of wind farm (WF) is consideredone of the major serious issues for developing power sys-tems based on renewable energy sources. A comparisonof dynamic performances for FACTS devices like STAT-COM and SVC is presented in this paper. Such devicesare employed to stabilize WF integrated with multi-machinepower system (WFMPS). Fuzzy logic controller (FLC) hasbeen employed for controlling FACTS devices to enhanceWFMPS performance under different abnormal conditions.Three-phase to ground short circuit fault has been takeninto consideration in different locations of system. MAT-LAB/Simulink is used to perform the modeling and sim-ulation of WFMPS. The obtained results demonstrate theeffect of both SVC- and STATCOM-based FLC on dampingthe system oscillations and improving the system dynamicperformance in the post-fault duration. The comparison con-firms superior dynamic performance and fast fault recoveryof STATCOM-based FLC through different fault conditionscompared with SVC-based FLC.

Keywords Wind energy system · STATCOM · SVC andfuzzy logic control

B Hegazy [email protected]

1 Electrical Engineering Department, Faculty of Engineering,Minia University, Minya, Egypt

2 College of Engineering at Wadi Aldawaser, Prince Sattam binAbdulaziz University, P.O. Box 54, Wadi Aldawaser 11991,Saudi Arabia

1 Introduction

Renewable energy systems that do not need fuel and do notaffect the environment have been considered an essential sig-nificance due to scarcity of fossil fuel [1,2]. Furthermore,increasing of electrical energy demand and growing envi-ronmental concerns. Wind power is the primary source inEurope and USA in 2015. 63 GW has been added during2015; therefore, the total installed capacity of wind energysystems (WESs) reaches up to 433 GW around the world.Figure 1 displays worldwide wind power capacity and yearlyadditions during the period from 2005 into 2015. The top 10countries,which have the highest capacity ofWESs, are illus-trated in Fig. 2. WESs are playing a main role for supplyingelectricity demand in many countries such as Denmark (42%of demand in 2015), Germany (60%) and Uruguay (15.5%)[3]. Accordingly, impacts of WES on utility grid (UG) havea huge attention from researchers. Besides its high capacity,WES has been considered cost competitive and environmen-tally friendly [4]. SCIG, PMSG and DFIG are consideredessential types of wind power generators. DFIG is widelyused in the WES due to its flexibility in control and highefficiency [5].

Today, wind farm (WF) connected to grid is used fre-quently. Therefore, its transient and dynamic performanceenhancement became an essential issue and received a hugeinterest from researchers. The flexible AC transmission sys-tems (FACTS) can be employed for ensuring power qualityof WF. They are perfect compensators for solving systemtroubles due to nonlinear loads and abnormal conditions.It has been considered the best solution to enhance perfor-mance of WF fed to UG as well as multi-machine systems.FACTS devices include different configurations like STAT-COM, SVC and UPFC [6]. STATCOM can generate andabsorb reactive power for controlling parameters of WES.

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Fig. 1 Worldwide wind power capacity and yearly additions during the period from 2005 into 2015

Fig. 2 The top 10 countries, which have the highest capacity of WESs, 2015

Improving performance of WES connected to UG has beenstudied for many years using FACTS devices for both steadyand transient states [7,8]. Many conventional techniques areproposed to control FACTS devices like PI and PID con-trollers. The shortage in the conventional control approachcharacteristics is their limited performance to a pre-scribedoperating conditions. Fuzzy logic control (FLC) has beenconsidered applicable choice commonly employed in differ-ent applications.

In this study, WF has been fed to the well-knownthree-machine nine-bus western system coordinate coun-cil (WSCC) power system. STATCOM and SVC are usedfor enhancing stability performance of WF. For investi-gating feasibility of proposed technique, a three-phase to

ground short circuit fault has taken into consideration for100 ms. in three different locations of the system. The dura-tion fault is increased to indicate their effect on the systemresponse.

2 Literature review

Several studies have been performed to enhance performanceof WES. Direct algorithms that can be employed for enhanc-ing dynamic stability of WES fed to UG-based SCIG havebeen presented [9]. Integration of SVC with WF to enhancevoltage and transient stability has been studied in [10,11].STATCOM has been used for improving generator output

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voltage and power under variable speed operation [12]. Alsoit is integrated with WF fed to UG to improve the dynamicand transient performances [13,14]. STATCOM is coupledwith WF-based FSIG for enhancing the voltage stability.STATCOM with capacity 10 MVA is used with WF in Bon-neville power administration system. Results of integrationindicated that STATCOM is an effective approach for reduc-ing voltage collapse due to gird disturbances. Furthermore, itimproves fault ride-through (FRT) capability of WF [15]. Anew control technique-based STATCOM has been proposedfor enhancing FRT capability of fixed speed IGs in [16]. Theintegration between a neurofuzzy controllers (NFC) tech-nique with STATCOM and SSSC for recovery of IGs fromsevere system disturbances and to stabilize wind generatorfed to UG has been presented in [17]. STATCOM has beenemployed for mitigating voltage fluctuations and improv-ing FRT capability of WF-based FSIG. The results indicatedthat the STATCOM has a significant effect on improvingthe power quality and enhancing FRT of WF [18]. Stabi-lizing of a WES using DFIGs or employing STATCOMunder changing of wind speed and grid fault is investi-gated. The results indicated that the WES can efficientlystabilize with both systems [19]. The effect of installingSTATCOM on improving the stability of FSIG-based WEShas been performed under different fault conditions [17].Various techniques are used by Saxena and Kumar [20] inorder to determine the parameters of STATCOM. Such tech-niques include GA, ANN and ANFIS. The performance ofSTATCOM is studied with changing wind speed. Further-more, the performance is evaluatedunder dynamic conditionsvia composite load. Results of the investigation indicatedthat the overall performance of STATCOM is enhanced andconfirms the superiority proposed algorithms over classicaltechniques under large disturbances [20]. Laouera et al. [21]studied the role of STATCOM with wind farm. The effectof reactive power injection for maintaining voltage withinthe limit has been investigated. MATLAB simulation hasbeen used to perform the considered analysis [21]. Ahsanand Siddiqui [22] investigated the effect of the integrationbetween 12 MW wind farm-based SCIG and STATCOMwith normal and abnormal conditions. The results indicatedthat STATCOM regulates fast changing of the voltage. Fur-thermore, STATCOM supplies active power for the period ofsuper-capacitor fault condition [22]. Das et al. [23] presenteda comparison between STATCOM and SVC for enhance-ment of voltage security. The results show the STATCOMis better than SVC for ensuring voltage security. Further-more, it can decrease reactive power line loss significantly[21]. The evaluation and comparison of STATCOM againstSVC that used for controlling and enhance the voltage profilehave been carried out by Rostami et al. [24]. The perfor-mance is evaluated under two different conditions. Suchconditions include 20% increasing in the load demand and

also symmetrical short circuit fault [24]. Cakir and Rad-man [25] investigated determining the optimum placementfor STATCOM and SVC for damping out the oscillationsvia residue technique. IEEE 14- Bus system has been mod-eled via PSAT software for demonstrating the performanceof studied method. The obtained results indicated that STAT-COM is more effective than SVC to damp out the oscillation.Pereira et al. [26] investigated the impact of STATCOM andSVC in dynamic voltage stability under low-voltage ridethrough. The considered WF includes WTGs-based FSIG.Moreover, various load models have been considered. Theoptimal location of STATCOM in multi-machine power sys-tem has been performed based on optimization technique.Such techniques include Cuckoo Search [27] and Imperi-alist competitive algorithm [28]. The STATCOM has beenemployed for achieving damping enhancement of wind farmconnected with a multi-machine system [29]. The proposedsystem includes a hybrid PID plus fuzzy controller. Theobtained results showed that hybrid PID plus FLC are betterthan for PID only to improve the stability of the stud-ied system [28]. In [30], a STATCOM has been employedfor improving the voltage profile at load bus in micro-grid.

3 System configuration

The case study has been applied to a commonly used WSCCsystem. Figure 3 illustrates the single line diagram of stud-ied system configuration and its parameters [31]. WSCCincludes 3-machines 9-bus and fed to three loads. MAT-LAB/Simulink has been employed to perform the modelingand simulation of studied system. A WF-based DFIG isfed to WSCC system instead of generator number 3 withthe same generated power. WF consists of three differentgroups each one has 20 turbine with capacity of 1.5 MW.Each group is connected to 575/13.8 KV transformer. Therating of each FACTS device is 100 MVAR. The specifica-tions of wind turbine are shown in Table 1. The descriptionof models for the studied system is described in next sec-tions.

4 DFIG-based wind farm

The integration of variable speed wind turbine and DFIGis received increasing attention. DFIG has numerous meritscompared with SCIG and PMSG such merits as high effi-ciency and flexible control and furthermore fast active andreactive power control and reduced mechanical stresses. Thestator of DFIG is directly fed to 3-phase system, whereas itsrotor is fed toUGvia transformer and back-to-back converteras illustrated in Fig. 4. The essential parts of the converter are

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Fig. 3 Studied power system configuration (60 Hz, 100 MVA Base)

Table 1 Parameters of windturbine-based DFIG Generator power (MW) 1.5/0.9 Friction factor (p.u.) 0.01

Phase to phase voltage (V ) 575 DC-Link voltage 1200

Stator resistance (p.u.) 0.00706 DC-Link capacitor 0.01

Rotor resistance (p.u.) 0.005 DC Battery capacity kWh 900

Stator leakage inductance (p.u.) 0.171 DC voltage regulator gains [Kp, Ki] [0.002 0.05]

Rotor leakage inductance (p.u.) 0.156 GSC regulator gains [Kp, Ki] [2.5 500]

Magnetizing inductance (p.u.) 2.9 RSC regulator gains [Kp, Ki] [0.3 8]

Base frequency (Hz) 60 Pairs of poles 3

rotor side converter (RSC), grid side converter (GSC), anddc-link capacitor [32]. The active and reactive power can becontrolled via RSC. On the other hand, GSC controls DC-link voltage. Furthermore, it ensures operation of converterwith a unity power factor.

The power converter takes about 20–30% of the totalpower for achieving full control of the generator. Accord-ingly, converter costs decreased and also power lossesminimized compared with fully variable speed WES witha full-rated converter.

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Fig. 4 Wind turbine-basedDFIG

5 Fuzzy logic control technique

Identification of PI controller parameters in order to enhanceSVCorSTATCOMperformance cannot be simplydone. FLChas been considered one of the nonlinear and robust controltechniques based on expert knowledge, and there is no needto have the accurate model of the system [33,34]. Fuzzifica-tion module, rule base, inference engine and defuzzificationmodule are considered as essential elements of FLC [33].In this work, Mamdani’s type FLC has been selected sinceit has some merits over Sugeno type. It is the intuitive,widespread acceptance and well suited to human input. Itcan easily implemented for controlling SVC susceptance (β)and STATCOM quadratic axis reference current (I ∗

q ). Theinput to FLC is the error voltage signal and change of error.FLC output is used to integrate (Iq ) in case of STATCOMand susceptance for SVC. The inputs and output member-ship function consists of seven linguistics of triangular type.The linguistic variables E , CE, and the controller outputwill take on the following linguistic values: (NB) = Nega-tive Big; (NM) = Negative Medium; (NS) = Negative Small;(Z) = Zero; (PS) = Positive Small; (PM) = Positive Medium;(PB) = Positive Big. The FLC rules and membership func-tions are illustrated in Table 2 and Fig. 5, respectively [35]. Ithas two inputs each has seven linguistics that produce forty-nine possibilities or rules. When applying the rules on theFLC input, the output is introduced that may be expressedby the FLC surface as shown in Fig. 6.

6 Static VAR compensator based FLC

SVCs can generate or absorb reactive power. As illustratedin Fig. 7, SVC configuration can be represented by thyris-tor switched reactor (TSR), thyristor control reactor (TCR),

Table 2 FLC rules

E CE

NB NM NS Z PS PM PB

NB NB NB NB NB NM NS Z

NM NB NB NB NM NS Z PS

NS NB NB NM NS Z PS PM

Z NB NM NS Z PS PM PB

PS NM NS Z PS PM PB PB

PM NS Z PS PM PB PB PB

PB Z PS PM PB PB PB PB

thyristor switched capacitor (TSC) or a parallel combinationof these configurations. The output reactive power is con-trolled via varyingfiring angle ofSCR.TCR inductive currentis controlled through a continuous control of firing angle [36].There are many important reasons for installing SVCs suchas improving dynamic voltage control [37]. Furthermore, itcan increase transferred power in case of low-voltage condi-tions. Accordingly, SVC decreases undesirable fault impactsupon generator for maintaining it in service. Voltage con-trol system is the principal basis of SVC that provides thesignal for firing thyristors for changing SVC’s susceptancevalue for keeping system voltage with required level [38].The operating principle of SVCwas explained and discussedin numerous papers [36–38].

Figure 8 shows the MATLAB/Simulink model of SVC-based FLC model. The system voltage is measured thencompared with reference voltage. After that error signal andits change are fed to FLC controller. The FLC output isemployed for generating the SVC’s susceptance (β). Theoutput is used to regulate the PWM inverter through varyingthe firing angle. This gives the appropriate reactive power[37,38].

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Fig. 5 FLC membershipfunctions; a Error signal bchange of Error c the outputsignal

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Fig. 6 Surface of FLC

Fig. 7 SVC configuration model

7 STATCOM-based FLC

STATCOM has been considered as one of shunt-connectedreactive power compensation devices. It is a solid-stateswitching converter supplied by energy source capacitor orenergy-storage device at its input terminals [39]. The outputlagging or leading reactive current is controlled independentof the AC system voltage. It consists of a voltage sourceconverter (VSC) which can produce 3-phase voltages. VSCwas driven via a dc storage capacitor. Schematic diagram ofSTATCOM construction is illustrated in Fig. 9 [40].

STATCOM can be employed for many purposes suchas filtering harmonics and improving stability. Moreover, itdeals voltage collapse. Integration of STATCOM with windturbine gives it a better performance and stabilizes the system[41]. Transfer reactive power amongSTATCOMand any sys-

Fig. 8 MATLAB/Simulink model of SVC-based FLC

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Fig. 9 Configuration model of STATCOM

tem achieved via controlling STATCOM magnitude voltagebased on the following relation [39,42];

Qc = V 21

Xtr− V1Vs

Xtrcos(θs − θ0) = V 2

1 − V1VS

Xtr

where;

Qc reactive power exchange between STATCOM andUG;

V1 UG voltage;Vs STATCOM output terminal voltage;Xtr Transformer leakage reactance.

The Simulink model for STATCOM-based FLC is shownin Fig. 10. The system voltage is measured at system pointof common coupling then compared with reference value.Then, the error signal and change of error are fed to FLC thatprovides the reactive reference current Iqre f . The STATCOMreactive current Iq is compared with Iqre f , and the output ofthe current regulator is the angle phase shift of the PWMinverter.

8 Analysis and discussion of simulation results

The WSCC power system considered in Fig. 3 has beenintegrated with FACTS devices (SVC, STATCOM) basedon fuzzy controller. Figure 11 shows a MATLAB/Simulinkmodel of studied system. For testing the system performance,a 3-phase to ground short circuit fault is applied at three dif-ferent locations. Such locations are selected to be positionedat the connection point of wind farmwithWSCC system, thesecond point located near to WF and the third one is sited farfromWF. Accordingly, the locations of faults are positionedat wind farm bus (B3), at system bus (B9) and at load 3 atbus (B8). A comparative study is held for investigating effectof STATCOM and SVC-based FLC on the system stabilityduring disturbance. The detailed analysis of three differentcases is explained as below:

8.1 Fault located at WF bus 3

A 3-phase to ground short circuit has been occurred at windfarm Bus 3. The fault duration was 0.1 s. it occurs at 1.0 sand cleared at 1.1 s. Figure 12 displays the dynamic timeresponse of SVC and STATCOM reactive power and theircontrol signals. From this figure, it can be noted that bothSVC and STATCOM supplied the same amount of reactivepower. The output reactive power of STATCOM is smoothwith faster response. On the other hand, SVC output is sharpwith much more ripples. The current quadratic axis compo-nent (Iq ) of STATCOMhas been quickly varied with reactivepower demand compared to SVC susceptance (β).

The dynamic time response of voltage, current, active,and reactive power of generator 1 is shown in Fig. 13. Onecan be noted that the system is unstable for a short periodbecause the fault is far away from this bus. The systemusing FACTs devices reaches steady state within 50%of time

Fig. 10 MATLAB/Simulink model of STATCOM-Based FLC

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Fig. 11 Simulink model of overall studied system

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Fig. 12 Dynamic time response of SVC and STATCOM reactive power

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Fig. 13 Dynamic time response of voltage, current, active and reactive power of generator 1

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Fig. 14 Dynamic time response of voltage, current, active and reactive power of Gen. 2

taken by the system without using FACTs devices. Further-more, the system without FACTS devices has higher overshooting.

The dynamic time response of voltage, current, active, andreactive power of generator 2 and wind farm is illustrated inFigs. 14 and 15, respectively. From Fig. 14, it can be noted

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Fig. 15 Dynamic time response of voltage, current, active and reactive power of wind farm

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Fig. 16 Dynamic time response of voltage at Buses B6, B7, B8 and B9

that the system using FACTs devices reaches steady statequickly (0.09 s). The system without FACTs devices takeslonger time to be stable (0.15 s).

From Fig. 15, it can be concluded that STATCOM andSVC have better time response. They have lower over shoot-ing (9%) and the system without facts has higher over

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SVC FLCSTATCOM FLC

no FACTs

SVC FLCSTATCOM FLC

no FACTs

SVC FLC

STATCOM FLC

no FACTs

SVC FLC

STATCOM FLC

Fig. 17 Dynamic time response of voltage, current, active and reactive power of generator 1

shooting (17%). Also the system without FACTs devicestakes too long time to be stable again (0.18 s) compared tosystem with SVC and STATCOM (0.07 s).

The dynamic time response of voltage of system BusesB6, B7, B8 and B9 is illustrated in Fig. 16. For the figure,it can found that the time response of all system Buses volt-age improved by FACTs devices. The system with FACTSdevices has smaller settling time and overshooting. Further-more, STATCOM provides better performance than SVC.

8.2 Fault located at bus 8

The dynamic time response of voltage, current, active, andreactive power of machine number one is illustrated inFig. 17.

From Fig. 17, one can note that the system is unstable fora short period because the fault is far away from this bus.The system using FACTs devices reaches steady state within(0.07 s) almost half of time taken by the systemwithout usingFACTs devices (0.12 s). Furthermore, the system withoutFACTS devices has higher over shooting (3%) compared tosystem with FACTs (1%). STATCOM provides much betterperformance than SVC. The dynamic time response of volt-age, current, active and reactive power of machine numbertwo is illustrated in Fig. 18. From Fig. 18, one can notedthat FACTs devices improve overall system characteristics;the system has smooth and fast response. It can be noted

that the system using FACTs devices reaches steady statequickly (0.08 s). The system without FACTs devices takeslonger time to be stable (0.15 s). Furthermore, the response ofthe system without FACTS devices is sharp with much moreripples. Also, it has higher over shooting (5%) compared tosystem with FACTs devices (1%).

Figure 19 displays the dynamic time response of voltage,current, active, and reactive power of WF. It can be con-cluded that STATCOM and SVC have better time response.They have lower over shooting (9%), and the system with-out FACTs has higher over shooting (16%). Also the systemwithout FACTs devices takes too long time to be stable again(0.16 s) compared to system with FACTs devices (0.09 s).

The dynamic time response of voltage of system BusesB6, B7, B8, and B9 is illustrated in Fig. 20. It can concludethat the time response of all system buses voltage improvedby FACTs devices. The system connected to FACTS deviceshas smaller settling time and overshooting.

8.3 Fault located at bus 9

A100-ms temporary three-phase short circuit is applied after1.0 s at Bus 9. Fig. 21 displays the dynamic time responseof voltage, current, active, and reactive power of generator1. From this figure, it can be noted that the system usingFACTs devices reaches steady state quickly (0.07 s). Thesystem without FACTs devices takes longer time to be stable

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Fig. 18 Dynamic time response of generator 2 parameters

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Fig. 19 Dynamic time response of WF parameters

(0.17 s). Also the over shooting value is higher for sys-tem without FACTS (4%) compared to system with FACTs(2%).

Figure 22 displays the dynamic time response of voltage,current, active, and reactive power of machine number two.It can be concluded that STATCOM and SVC have better

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olta

ge b

us9

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Fig. 20 Dynamic time response of voltage at Buses B6, B7, B8, and B9

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time response. They have lower over shooting (1%), and thesystem without facts has higher over shooting (4%). Alsothe system without FACTs devices takes too long time to

be stable again (0.15 s) compared to system with SVC andSTATCOM (0.08).

Figure 23 displays the dynamic time response of volt-age, current, active, and reactive power of machine number

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tage

(pu)

1.00 1.10 1.20 1.30 1.40

0.8

1

1.2

Time (sec)

Cur

rent

(pu)

1.00 1.10 1.20 1.30 1.40

-1

-0.5

0

Time (sec)

Act

ive

Pow

er(p

u)

1.00 1.10 1.20 1.30 1.40

-0.4

-0.2

0

0.2

0.4

Time (sec)

Rea

ctiv

e P

ower

(pu)






1.00 1.10 1.20 1.30 1.40

0.2

0.4

0.6

0.8

1

1.2

Time (sec)

Vol

tage

(pu)


1.00 1.10 1.20 1.30 1.40

0.5

1

1.5

Time (sec)

Cur

rent

(pu)


1.00 1.10 1.20 1.30 1.40

-0.2

0

0.2

0.4

0.6

Time (sec)

Act

ive

Pow

er(p

u)


1.00 1.10 1.20 1.30 1.40

0

0.5

1

Time (sec)

Rea

ctiv

e P

ower

(pu)


Fig. 23 Dynamic time response of WF parameters

three. In this state, the effectiveness of FACTs device isvery clear because the fault is very close to WES. It canbe concluded that STATCOM and SVC have better time

response. They have lower over shooting (9%), and the sys-tem without FACTs has higher over shooting (17%). Alsothe system without FACTs devices takes too long time to be

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Electr Eng

stable again (0.15 s) compared to systemwith FACTs devices(0.08 s).

9 Conclusion

In this paper, the performance of wind farm-based DFIGfed to WSCC multi-machine power system has been eval-uated. The studied system subjected to sever three lines toground fault at various locations. Such locations are selectedto be positioned at the connection point of wind farm withWSCC system, the second point is located near to windfarm, and the third one is sited far from wind farm. A Mam-dani’s type FLC has been employed for controlling SVCand STATCOM that connected at wind farm. A compari-son study between SVC and STATCOM has been carriedout. The results of comparison reveal that FLC has a bet-ter performance with less overshoot during transient faults.Both SVC and STATCOM supplied the power system withreactive power under fault condition. FLC controller pro-vides better damping oscillation with a reduced settling time,fast and smooth response under fault conditions. Moreover,both SVC and STATCOM provide rapid recovery of termi-nal voltage after fault clearance. Finally, it can be said thatSTATCOM provides better performance than SVC, espe-cially if such devices controlled via fractional-order-basedfuzzy logic control. The integration between FLC robustnessand the accuracy of fractional-order will enhance the over-all system performance in comparison with the conventionalones.

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A comprehensive comparison of STATCOM versus SVC-based