IEEE ISIE 2006, July 9-12, 2006, Montreal, Quebec, Canada

Application of DSTATCOM for Mitigation ofVoltage Sag for Motor Loads in Isolated Distribution

Bhim SinghDepartment of Electrical Engg.Indian Institute of TechnologyNew Delhi, 110016, Indiabhimsinghr@gmail.com

SystemsA. Adya, A.P.Mittal, J.R.P.Gupta B.N.SinghDepartment of Instrumentation and Control Engg. Deptt. of Electrical Engg. and Computer ScienceNetaji Subhas Institute of Technology New OrleansNew Delhi-1 10075, India LA 70118, USAalkaadya@gmail.com, mittalap@yahoo.com, jrp83 @yahoo.com

Abstract This paper deals with one of the potential applicationsof distribution static compensator (DSTATCOM) to industrialsystems for mitigation of voltage dip problem. The dip in voltageis generally encountered during the starting of an inductionmotor. Isolated distribution systems are comparatively not as stiffas grid systems; so large starting currents and objectionablevoltage drop during starting of an induction motor could becritical for the entire system. DSTATCOM is one effectivesolution for isolated power systems facing such power qualityproblems. The model of DSTATCOM connected in shuntconfiguration to such an isolated system (3phase, 42.5kVAalternator) feeding dynamic motor loads is developed usingSimulink and PSB of MATLAB software. Simulated resultsdemonstrate that DSTATCOM can be a considered as a viablesolution for solving such voltage dip problems.

Keywords-DSTATCOM; voltage regulation; induction motor.

I. INTRODUCTIONImproved power quality is the driving force for modernindustry. Consumer awareness regarding reliable power supplyhas increased tremendously in the last decade. This has given anew thrust to the development of small distributed generation.Small isolated DG sets have the capability to feed local loadsand thus improve reliability of power with low capitalinvestment. These systems are also gaining increasedimportance in isolated areas where transmission usingoverhead conductors or cables is unfeasible or prohibitive dueto excessive cost. Small generation systems in hilly terrains,islands, off shore plants, power distribution in rural areas,aircrafts etc. can be effectively utilized even in developingcountries. However, these DG sets may have to be de-rated ifinduction motor loads are simultaneously started. One effectiveoption is to use DSTATCOM in shunt configuration with themain system so that the full capacity of generating sets iseffectively utilized. DSTATCOM employs a voltage sourceconverter (VSC) and it internally generates capacitive andinductive reactive power. Its control is very fast and has thecapability to provide adequate reactive compensation to thesystem [1-4]. DSTATCOM can be effectively used to regulatevoltage for one large rating motor or a series of small inductionmotors starting simultaneously. Induction motor loads drawlarge starting currents (5- 6times) of full rated current [5] andmay affect working of sensitive loads.

Thyristor based systems have been initially proposed forreactive power compensation and used for voltage flickerreduction due to arc furnace loads [6-7]. However, due todisadvantages of passive devices such as large size, fixedcompensation, possibility of resonance etc., the use of newcompensators such as DSTATCOM for solving power qualityproblems is growing. Various authors [8-14] have reported theuse of DSTATCOM for solving power quality problems due tovoltage sags, flickers, swell etc have been reported. Akagi et al[15] have proposed instantaneous reactive power compensatorusing switching devices. Singh et al [16-17] have listedmultifunctional capabilities of STATCOM and presentedindirect current control scheme for DSTATCOM. Singh et al[18] have designed DSTATCOM for self excited inductiongenerator which has inherent poor voltage regulation.Simulation of DSTATCOM and custom power devices havebeen carried out using standard software such as PSCAD/EMTDC, SABER, PSPICE, MATLAB etc [19-20].In this paper, an application of DSTATCOM to isolatedgeneration system feeding dynamic loads is presented. The useof DSTATCOM is to provide efficient voltage regulationduring short duration of induction motor starting and thusprevents large voltage dips. DSTATCOM can be effectivelyused to regulate voltage for one large rating motor or a seriesof small induction motors starting simultaneously.

II. SYSTEM CONFIGURATIONFig. la shows the schematic diagram of DSTATCOM forproviding voltage regulation. A three-phase alternator of 42.5kVA, 50 Hz, 400V (L-L) rating feeds power to isolateddistribution system. The alternator is coupled to the dieselengine with governor as prime mover. The load considered onthe system represents an induction motor load. Thesynchronous machine output voltage and frequency are used asfeedback inputs to a control system, which consists of thediesel engine with governor as well as an excitation system.Fig.lb shows the basic diagram of DSTATCOM connected asshunt compensator. It consists of a three-phase, currentcontrolled voltage source converter (CC-VSC) and anelectrolytic DC capacitor. The DC bus capacitor is used toprovide a self supporting DC bus. AC output terminals of theDSTATCOM are connected through filter reactance or inpractical case, by the reactance of the connecting transformer.

1-4244-0497-5/06/$20.00 2006 IEEE 1 806

DSTATCOM provides fast and efficient reactive powercompensation.

III. CONTROL SCHEMEFig.2 shows the control scheme for voltage regulationpurpose. Two PI controllers are used. One PI controller isrealized over the sensed and reference values of dc busvoltage of the DSTATCOM. The second PI controller isrealized over the sensed and reference values of ac voltage at

* R_ Ln V, X

DieselEngine withGovernorControl

42.5 kVAalternator Motor Load

V

R,, L,DC

Capacitor

DSTATCOM

Fig. la Schematic diagram of DSTATCOM system connected to anisolated alternator feeding motor loads.

quadrature components results in the three-phase referencesupply currents (is,,, isbr, and iscr). These three-phasereference supply currents are computed using three-phasesupply voltages and dc bus voltage of the DSTATCOM.

(a) Computation of In-Phase Components of ReferenceSupply CurrentsThe amplitude of in-phase component of reference supplycurrents (Ispdr) is computed using first PI controller over theaverage value of dc bus voltage of the DSTATCOM and itsreference counterpart.ispdr(n) = Ispdr(n-1) + Kpd{Vde(n)- Vde(n-1)} + Kid Vde(n) (1)where vde(n) = vdcr- vdca(n) denotes the error in vdc calculatedover reference vdcr and average value of vdc and Kpd and Kidare proportional and integral gains of the dc bus voltage PIcontroller.The output of this PI controller is taken as the amplitude ofin-phase component of the reference supply currents. Three-phase, in-phase components of the reference supply currentsare computed using their amplitude and in-phase unit currentvectors derived from the supply voltages and amplitude ofsupply voltage which is computed as:vtm2 = 2/3(vta' + vtb + Vtc') (2)The unit vectors (ua, Ub, uc ) are calculated as:Ua=Vta I vtm,Ub=Vtb/ Vtm,uc=vtC/ vtm 3The in-phase magnitudes of reference currents (isadr, isbdr, iscdr)are calculated as:isadr = lspdr Uaisbdr = lspdr Ubiscdr = lspdr Uc (4)

S5

b YLCd Vdc

RC) Lc 1 1 'S6 S2 S4

Fig.lb Schematic diagram of 3-legged DSTATCOM system

PCC. The output of the first PI controller (Ispdr) is consideredas amplitude of in-phase components of reference supplycurrents and the output of second PI controller (Ispqr) isconsidered as amplitude of quadrature components ofreference supply currents. A set of in-phase unit vectors (ua,ub and uj) are computed by dividing the terminal voltages(vta, vtb and vtc) by their amplitude (vt.). Another set ofthree-phase quadrature unit current vectors (wa. wb and wj)are derived from in-phase unit current vectors(ua, ub anduj).The multiplication of in-phase amplitude with in-phaseunit current vectors results in the in-phase components (isadr,isbdr and iscdr) of three-phase reference supply currents andsimilarly multiplication of quadrature amplitude withquadrature unit current vectors results in the quadraturecomponents (isaqr, isbqr and iscqr)of three-phase referencesupply currents. Algebraic sum of these in-phase and

(b) Computation of Quadrature Components ofReference Supply CurrentsThe amplitude of quadrature component of reference supplycurrents (Ispqr) is computed using another PI controller overthe average value of amplitude of supply voltage and itsreference counterpart.spqr(n) Ispqr(n-l) + Kpq{vae(n)- vae(n 1)} + Kiq vae(n) (5)where vae(n)= Vtmr- vtm(n) denotes the error in Vtm calculatedover reference Vtmr and average value of Vtm and Kpq andKiq are the proportional and integral gains of the second PIcontroller.The quadrature unit current vectors (wa, wb, wc) are derivedfrom in-phase unit current vectors (ua, ub, uc ) as:wa={-ub + uc}I/ (3)1/2wb={ua(3) + (Ub-uc) }/ 2(3)wC={-ua(3) + (Ub-uc)}/ 2(3)1/2 (6)(c) Computation of Total Reference Supply CurrentsOnce the total reference currents are obtained by theaddition of respective in-phase and quadrature currentcomponents as:isar = isadr + isaqr (8)isbr = isbdr + isbqr (9)iscr = iscdr + iscqr (10)

A PWM hysteresis controller is applied over the sensed (isa,isb and isc) and the reference values of supply currents (is5,isbr and iscr) to generate six gating pulses for the six IGBTswitches of the DSTATCOM.

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i'a itb j,

VtI,

vtm.r \>/ isabcr 6 gating

AL ~~~~~~~~~~pulsestoX ALjs DSTATCOM/ isidabcr / / isabc

Inphase reference Pi contr VdcrcurrentsP1cnrle

Fig. 2 Schematic diagram of DSTATCOM Control scheme

Fig.3 MATLAB based model of Power Circuit

Fig.4 MATLAB based model of Control Scheme for generation of reference currents

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MeasurementDem ux

IV. MATLAB BASED MODELING OF SYSTEMModel of the STATCOM including power distributionsystem network and its controller is developed in MATLABenvironment with Simulink and PSB toolboxes.A. Power Circuit

Fig.3 shows the developed model of power circuit of systemusing MATLAB and Power-system block-set (PSB). Thesynchronous generator is driven by a diesel engine set. Asalient pole 42.5kVA, 2-pole, 50 Hz, 400V rms voltage threephase synchronous machine is modeled as an isolatedalternator. A typical IEEE Type-I synchronous machine withvoltage regulator combined to an exciter is used. A 7.5kWinduction motor is connected at the other end of alternator.An IGBT based PWM voltage source inverter asDSTATCOM is implemented using Universal bridge blockfrom Power Electronics subset of Power System Blockset. Itis connected in shunt with the main system via transformerimpedence (R7,Lj). The current regulator block uses voltageinputs and generates gating pulses for IGBT switches ofVSC. The paramters of the alternator system, motor load,DSTATCOM, PI controllers are given in Appendix.

B. Control Scheme

Fig.4 shows the control scheme model of DSTATCOMdeveloped using MATLAB. This figure shows the generationof terminal voltage, unit in-phase current templates and unitquadrature current templates. These are used to generate in-phase components of reference currents and quadraturecomponents of reference currents. The in-phase componentsreference currents are responsible for power factor correctionof load and the quadrature components of supply referencecurrents are responsible to regulate the AC system at PCC.The reference supply currents are generated using theindirect current control scheme as illustrated using equations(1)- (10).A carrier-less hysteresis PWM controller is employed overthe sensed supply currents (isa, isb, isc) and instantaneousreference supply currents (isai, isbr, iscr) to generate six gatingpulses for DSTATCOM. The PWM current controllercontrols supply currents in a band around the desiredreference current values. If the current in phase 'A' is lessthan reference current in that phase, then upper IGBT for leg'a' is 'OFF' and lower IGBT is 'ON'. Similar logic isapplied to the other two legs. The controller controls thesupply currents in a band (hb) around the desired referencecurrent values. The hysteresis controller generatesappropriate switching pulses for six IGBTs of the VSIinverter.

V. PERFORMANCEOFDSTATCOM SYSTEMPerformance of DSTATCOM for power-factor correction,voltage regulation and harmonic reduction along with loadbalancing is studied. The performance of the model isanalyzed under various conditions.

A. Performance of Isolated Alternator System with MotorLoads without DSTATCOM

Fig.5 shows the response of alternator system with dynamicmotor load. This figure shows the supply voltage (vs),voltage at PCC (vt), supply currents (is), load currents (ij),and voltage (vtm) at the PCC. The motor load is applied att=0.3sec and the simulated results show that voltage dipsinstantaneously. Voltage (vtm) dips from the reference valueof 328V to 250V which is 23.8% voltage dip. This largevoltage dip is encountered at the starting of induction motoras the motor draws 5-6 times the full load currents duringthis duration. The motor now develops rated speed and it isput on full load at t=0.48sec. However, the voltage dip isnow within limits as the motor is already started and isdrawing normal full rated current.

B. Performance of DSTATCOM with Alternator Systemfeeding Motor Loadsfor Voltage Regulation

Fig.6 shows the response of DSTATCOM system applied inshunt configuration to the alternator system feeding motorload. This figure shows the dynamic performance ofvariables such as supply voltage (vs), voltage at PCC (vt),supply currents (is), load currents (ij), DSTATCOM currents(ic), DC link voltage (vdc) and voltage at PCC (vtm). Themotor load is applied at t=0.3sec and it is observed that thevoltage at PCC dips. However, DSTATCOM system is ableto reduce the dip from 328V to 300V. Two PI controllers areused- one is to regulate the DC link voltage and the other oneis to regulate the ac terminal voltage at PCC. The momentaryvoltage dip is approx. 8% which is much less as compared tothe alternator system without DSTATCOM. The full load onthe motor is applied at t=0.48sec and the voltage at PCC isregulated nearly to reference value of 328V.

IV. CONCLUSIONSA model of isolated alternator system feeding motor loadshas been developed using PSB and Simulink of standardMATLAB software. Sudden application of an inductionmotor load results in large starting currents which results insudden dip in ac terminal voltage at PCC. The extent ofvoltage dip with and without DSTATCOM controller iscompared. The voltage dip is of the order of 23.8% withoutany controller. This dip is very large and it may affect thefunctioning of other sensitive equipment connected at PCC.Model of DSTATCOM system applied in shuntconfiguration has been developed. The DSTATCOM controlutilizes two PI controllers for regulating DC link voltage andalso the ac terminal voltage at PCC. The simulated resultshave shown that DSTATCOM application reduces themomentary dip to approximately 8% only. The voltage dipcan be further reduced by proper tuning of PI controllers anduse of fixed value of AC capacitors.

AppendixSystem parameters used in simulationAlternator system parameters: 42.5 kVA, 400V (L-L rms), 2 pole,50Hz, H=0. 1 157s,Stator: Rs=0.04808, L1=0.08, Lmd=2. 11, Lmq=0 93 (all in pu)Field: Rf=0.02662, Llfd=0.l582Dampers: Rkd=0.0754, Llkd=O.1098, Rkql=0.0731 1, Llkql=0.06414(all in p.u.)DSTATCOM parameters: R,= 0.1Q, L,=7.5mh,CdC=47OOgf, hb= 0.5 A

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500

0

500500

0

500200

0

-200200

-200

400

>-200

2000

1000

00.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7

Time(sec)

Fig.5 Performance of alternator system feeding motor loads without DSTATCOM

5000

500500

-500100

-100 L200

-200200

-200

S 900 -

> 700

5 400 _>;E200> 2000

Q- 1000 ,0 00.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65

Ti me(sec)

Fig.6 Performance of alternator system feeding motor loads with DSTATCOM

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0.7

PI Controller parameters: Kpd= 0.3, Kid= 0.8,Kpa=O.15, Kia= 1.5Motor load:Load parameters: 7.5kW, 3-phase, 415V, r,=0.6387, rr=0.451,Llk=0.004152, Ll,=0.004152, Lm=0.1486(all in pu)

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