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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, [email protected]
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, [email protected],
[email protected], 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
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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
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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
1809
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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
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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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