POWER SYSTEM PERFORMANCE ENHANCEMENT USING UNIFIED POWER FLOW CONTROLLER

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POWER SYSTEM PERFORMANCE ENHANCEMENT USING UNIFIED POWER

FLOW CONTROLLER

by

@Harlnder Sawhney,B.Eng.

A thesissubm ittedto theSchoolof Gra duate Studiesinpartial fulfi llmentof the

Requiremen tsof thedegreeof Masterof Engineeri ng

Facult y ofEngineeri ngand AppliedScience Memorial Universityof Newfound land

Sept emb er2002

St.Jo hn's Newfound la nd Canada

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Abst ract

Electricpower utilitiesin many countries aroundtheworld face deregulation andprivatization. Theutilities are often separatedinto generation.transmission and distributioncompanies in orderto encouragecompetitionand provide customers with the choiceof selecting theirelectrical energy provider .Environmentalconce rns, right-of-wayand cost have del ayedthe cons truct ionof new transmiss io nlines.The demand for electricpower has continued10 grow and thismust beme tbyincreased transferof powerthroughavailabletransmissio nlines.

The overall aim of theresearch presentedin this thesisis to examinethe applicationof Unified Power RowController(UPFe)in powersystem operation.

For thisdevice a generalmodel isderivedand used inpowersystemanalysis.This model is referred to as theinjectionmodelwhich is valid for load flow analysis.The model has been very helpful forunderstan ding the impactof UPFCon powersyste m operation.As a part ofUPFC applica tio n,Available Transfer Capability(ATC) has beenstudiedwithdifferenl methodsof calculatingATC.The impact ofthe UPFCin inc reasing the availabletransfercapability ofthe power systemhas been studied.

Test resultsusing differentpowersystemmodels are presentedthroughout the thesis to illustrate the effectiveness of UnifiedPowerFlo w Controller.

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Ack nowledgements

Many thanks to my father andmotherfor their prayers,love,and support.

Without themthis workwould never have comeinto existence. Alsothanks10 my brotherRavinder andmy two sistersPanninderandHarpreelfo r theirconstant encouragementduring mystudyin Canada.

Iwouldlike to thank andexpressmy indebtednessandheartiestgratitude to my supervisorDr.B.Jeyasurya for his constant advice,encouragement andguidance duringall stages of thisresearch.

Than ksare duetothe staff atCCAE andfacultyfor all theuseful discussi ons andsugges tions.

Advice andsupportprovided byMr.Harjeet SinghGillof ABBInc.,Sant a Clara,CA, U.S.Ainvariouswaysishighlyack no wledged.

Finally,I wouldliketo expressmy thanksto MemorialUniversity of Newfoundlandforthefinancialsupport. whichmade thisresearchpossible.

DedicatedTo MyParents

Sm t. BafwantKaur Sethi s.PrithiPalSing hSaw h ney

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II.

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List of Figures

1.1 Power flow inaTwo Bus System 1.2 Po wer flow afteraltering gener ation 1.3 Po wer Systemwitha controller

1.4 Lumpedelementrepresentat ionof alosstess transmission line I.5 Twomachine powersystemwithinductiveline

1.6 Power trans missionvs.anglecharac te ristic 10 1.7 Twomachinepowersystemwithaidealmid point reactive 12

Compensa tor

1.8 PowerTransmissio nvs. anglecharacteristic 13

1.9 Two machine power syste mwithseriescapacit ivecompensation 14 1.10 Powertransmissio nvs.anglecharacte ristic 15 1.11 Two machinepowersystemwith aphase shifter 16

1.12 Power transmissionvs.anglecharacteris tic 17

1.13 Fixed cap acitor-T hyrist orcontrolledreactor typeSVC 20 1.14 Thyristor Controlled Series Compensator Scheme 21

1.15 Schematic diagra m of a phaseshifte r 22

1.16 StaticSynchronousCompensator 24

1.17 Static Synchrono usSeries Compensator 25

1.18 Twomachine systemwiththe Unified PowerFlow Contro ller 26

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1.19 UnifiedPower Row Controller 27

2.1 Basiccircuitarrangementof UPFC 31

2.2 Phasor diagramshowing control capabilityof UPFC

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2.3 (a) Asim ple twomachinesystem

"

2.3 (b) RelatedVoltage Phasors 34

2.3(c ) Real andReactive powerversustransmissionangle 34 2.4 Twomach ine syste mwiththeUnifiedPo wer Flow Controller 35 25 Con cept ualrepresentation of UP FC in a two-BusPowerSystem 36 2.6 Controlregion of the attainable realpower P andreceivingend 40

reactivepowerdemand Q withaUPFCcontrolled trans mission lineat o=O°(a)0=300(b)o=6O"

(c )o=9(t(d)

2.7 CurrentSourcedConverter 41

2.8 VoltageSourced Converter 42

2.9 Circuitarrangement forSPWM 45

2.10 Circuitarrangement for generationof injectedvoltage 46

2.11 Plot ofinverter outp utwaveforms 47

3.1 Equivalentcircuitdiagramof a Series ControllableDevice 52 3.2 vectordiagram of the SeriesControl lableDevice 52 3.3 Re presentation ofthe seriesconnected voltagesource 53 3.4 Replacementof the series vo ltagesource byacurrentsource 54

3.5 Injection model ofthe seriespart of the UPFC 55

3.6 Injectionmodelofthe UPFC 56

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3.7 Flow chartfordeterm ining Loadflow solutionincluding the 63 UPFCmodel

4.1 FiveBusSystem 66

4.2 Varia tionofP against randy 69

4.3 variationofQ again st rand y 70

4.4 Single line diagra m of24 -8u 5Reli abili tyTe stSyst em 72

4.5 Sing le line diagram of the3D-Buslest System 75

4.6 Singlelinediagramofthe39-8 u5 powerSystem 78

5.1 Singleline diagramof6-bus power system 90

5.2 Sing lelinediagra mof6bussysteminnorm alstate 92 5.3 Singlelinediagra m of6 bus systemshowing line1-5operat ing 93

nearitsthermal limit.

5.4 Single line diagramof 6bus systemin norm al state 94 5.5 Single line diagra mof 6 bus system inoverlo ad ed state 95 5.6 Singlelinediagramofthe 24-8 us ReliabilityTestSystem 98 5.7 Singl elinediagramof the39-huspower system 102

5.8 Main structure of BCHydro powersystem 105

5.9 Geographic location of

Be

Hydrotransmission network ₁₀₆

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List of Tables

3.1 Modificati on ofJacobianmatrix 62

4.1 Compariso n between powerflowresultswithandwithout UPFC 67 (5-8 u5system)

4.2 IEEE 24 BusReliabilit y Systemgenerationdispatch 71 4.3 Co mpariso n betweenpower flow resul ts withandwithout UPFC 72

(24·8u5system)

4.4 Comparis onbetweenpowerflow resul ts with and withoutUPFC 76 (30. Bussystem)

4.5 New England39- Bus powersystemcomponent data 77

4.6 NewEngland39-8u$ power system base case 77

4.7 Comparis onbetwee npower flow resultswith andwithoutUPFC 79 (39-Bussystem)

5.1 6-bus power systemcomponent data 90

5.2 6-buspower system base case 90

5.3 ATeresults for 6-BusSystem 91

5.4 IEEE24 BusReliability Systemgener ationdispatc h 97 5.5 Base CaseATeResultsfor24 Buspower system 100

5.6 ATeResultswith UPFC 100

5.7 NewEngland 39-Bus powersystemcompo ne nt data IOJ

5.8 NewEngland 39-Bus power systembasecase 101

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5.9 Base CaseATeResultsfor 39- Bus power system 103

5.10 ATeResultswith UPFC 103

5.11 BCHydro mainpower plant summary 107

5.12 WSCC 8313-Bus powersystemcomponentdata 109

5.13 WSCC 8313-Buspowersystem base case 109

5.14 BCHydro 196-Buspower system component data 109

5.15 BCHydro196-Buspower systembase case 109

5.16 Base Case ATC Resultsfor196Buspower system III

5.17 ATC Results with UPFC III

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List of Abbreviations and Symbols

FAcrS Flexible A.C Transmission System TCR Thyristor Controlled Reactor TSC Thyristor Switched Capacitor SVC Static Var Compensator STATCOM: Static Synchronous Compensator TCSC Thyristor Controlled Series Capacitor SSSC Static Synchronous Series Compensator PAR Phase Angle Regulator UPFC UnifiedPower Flow Controller

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Chapter 1 Introduction to Flexible A.C Transm ission System

1.1 Introduction

Electric utility plannersare currentlyfaced witha major challenge in meeting the increasedload demand withhighreliability and minimuminvestmentin new transmission facilities.Laying of additional parallellines andacquiring necessary right of waysor raising systemoperatingvoltages may all be prohibitivefrom economical andother considera tions.

Optimalutilization of transmissionlines has thus become a strict requirement on energy systemsin viewof limited availabilityof transmissioncorridors. Withthe growingreq uirement of transmissio nof bulk power 10 expanding load centers over restricted right ofways,the need to usc transmissionfacilitiesinan optimum and efficientmanner is being increasingly felt(I).

Demand of electricalenergy continuesto grow steadily.Electricitygrid upgrade, and especially the construction ofnewtransmissionlines,cannot keeppace with the growingpower plant capacity and energy demand. Findingsuitableright of

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ways isparticularlydifficult in theindustrializedcountries, and gaining the necessary approvalistime consuming particularlydue toenvironmentalconsiderations.

Due tothese constraintsit has becomea greatchallengeto utilizethe existing powerlinesmore efficiently. This challengecanbeconsideredfromtwoaspects.In the first placethereis aneedtoimprovethe transientandsteadystalestabilityoflong distance high voltagetransmission lines. The powertransfercapabilityoflong transmission linesis limitedfrom bothsteady stateandtransientstabilitypointof view.Theotheraspectistheflexibility aderegulation energy marketrequires.Power systemoperation to ensure thattheelectricitysupplycontractscanbefulfilledina deregulatedmarket needs innovativefeatures.Thischapterdiscussesthelimitsfor power transferoveraninterconnected powersystem. Theprinciple of shunt and series compensationispresented next.Overview of different FlexibleA.CTransmission System (FACTS) Controllers and theirpotentialtoovercomesome of thelimitsin existing power system are also illustratedin thischapter.

1.2 Limitsto Power Flowin

a

Transmission System

Itisdesiredtoutilize thetransmissioncapacitytoits bestuse taking into account loadingcapabilityand contingencyconditions,butthereis alimit tothe loadingcapability of transmission lines.

The limits to powerflow over transmissionlines areclassified as [2]

Thermal Voltagedrop Limits Stability

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• LoopFlow

Thermal Limits:Thermal limitof a transmission line is a function of the temperature, environmentalconditions, physicalstructureof the conductor,and ground clearance.

Line losses convert electricalenergy to heat and heat weakens the power lines conductor. This "lost" electricalpower heats the powerlines andcauses the conductor to expand and the line to sag.At some temperature the conductor becomes soft enough to be permanently damaged by the line's weight.At a higher temperature the conductor will melt and break.Hence thennallimitimposes a limit on thepower flow through the transmission line.

Voltagedrop limits:As loadincreases,voltage atreceiving substationdecreases.For theequipmentsto operate properly,the voltage should not be allowed to fall below a specified value.Thislimits thepower transfer.

StabilityLimits:These are a number of stability concerns that limit the transmission

capability.Theseinclude:

Transient Stability

Steady Statestability Voltagecollapse Loop Row

TransientStab ility: During fault on a transmissionline, at a generatorstation, or at a substation there is a possibility of rotor angleincreasing veryrapidly.This leads 10 reductionin transfer of powerfrom generatingendto the sendingend. However,

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mechanical powerproduced by theturbine remains constantduringthefault, andthere is consequently animbalance between mechanicalpower input tothe generatorand electrical poweroutput, with mechanicalpower beinginexcess.Thisexcess mechanicalpoweris convertedto acceleratingpower and the generatorincreasesits rotationalvelocity.Thismay causethe generatortolose synchronismif the faultisnot clearedquickly.Theresultingsystemresponseinvolveslarge excursions of generator anglesand isinfluencedbythenonlinear power-anglerelationship.Thetransient stabilityof generator depends uponthe generator loading,faultclearingtime and generatorreactanceetc.[3J.

Steady StateStabi lity:II isdefined as the abilityof the powersystem10 maintain synchronismundersmalldisturbances.Such disturbancesoccurcontinuallyonthe system because ofsmall variations in loads andgeneration.The disturbancesare consideredsufficientlysmallforlinearization of system equations 10bepermissible forpurpose of analysis(3].

VoltageCollapse: A system entersa stateof voltageinstabilitywhena disturbance, increase in load demand.orchangeinsystem conditioncauses aprogressive and uncontrollabledropin voltage.Voltagecollapse isthe processbywhichthesequence of eventsaccompanyingvoltage instabilityleadsto low unacceptable voltageprofile in a significantpartofthepowersystem [3)

LoopFlow:In aninterconnectedpower system,powerflowing between a generator and a customermoves throughall lines connecting the two points.Power follows the path ofthe leastresistanceaccording to the Jawsof physics.Powerflow isinversely

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proportional to transmissionline impedanceratherthan current capacity of transmissionlines.Neighboringtransmissio nsystems are usually connected together in a largenetwor k.This resultsinan exchangeof powerbetweendifferentareas, and is termed as loop powerflow.Loop flows are difficu lt to control andcan damage the transmissio nequiprnents.Thisphenomenonis explained below witha simpleexample of a two bussystem.

Considera simplecase ofpower flow through twoparallellines asshownin Figurel.lbelow. Power flow is proport iona lto the inverse of the transmission line impedance.This law governingthe power flow suggeststhat theline withlo wer impedancemaybe overloadedandthe linewith higher impedance mightnot be fully loaded to itstransmissioncapacity.In Figure 1.1, generators attwo differentsites are sendingpower to the load throughtwoparallel lines.Line carryingpowerP IandP2 have ratingsof 75MW and 250MWrespectively .

lOOMW Generation PI= IOOMW

300MW

G'"O-1 I---_~

P2=200MW

-

Figure 1.1Powerflow in aTwoBus System

One of thegeneratorsis generating 30UMW andother100MWto meet theload demand of400 MW.Forthe situationshowninFigure1.1theline carryingpower PI is overloaded .Assuming there are no controldevicesto reroute powerbetweenthe

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lines,flow is primarily controlled by changingthepatternof generationoutput or frequentlyby reducing theload as shownin Figure1.2.

An alternative to sucha caseofoperation is alteringthe patternof generation,or one of the line carrying powerP2 couldbeinstalled with aload sharing device(Controller) . The availablegenerationsourcessupplya givenload for each controlareain themost economic manner inreal-timegeneratio n.Theobject iveis to minimize thetotalgenerationcost.The disadvantagesofaltering the generationis that it wouldbe no longer economicalfor the customer asthe cost of generatio nmay be high.By controlling the impedanceor phase angle,orthroughseries injection of appropriatevoltage in theline, powerflow canbecontrolledasdesired.

Figure 1.3 showsthat withthehelp of a controllerthe line overload canbe relieved,more power canbe pushedthrough theline resulting inthe linebeingused 10 itstransmission capacity.The overallCOS!of generationisalso reduced.

225MW

PJ=75 MW J75MW

Generation

-

Generatio n

&1

^Xl^X2=,05^=,IO

_~

^400MWLoad

-

Figure1.2Powerflow afteraltering generation

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PI=75MW

400MW Load

Figure1.3Power Systemwitha controller

1.2.1 Basicsof AC PowerTransmission

The main components of anac power transmissionsystemaregenerators, transmissionlines, distributionlines and loads,with their relatedauxiliaries and protectionequipment.The generatorsarc rotating synchronous machines.The transmission systemsare designed to operateathigh,medium, andlow,alternating voltages.The loadsmay be synchronous.non-synchronousandpassive,consuming generallybothreal andreactivepower.The modempowersystemisacomplex network of transmissionlines interconnecting allthe generatorstationsandallthe majorloadingpointsin thepowersystem.Theselines carry largeblock of power to meetthedemandsatvariouspartsofthe powersystem.

Alternating currenttransmissionJinesarecharacterizedby their distributed circuitparameters: the series resistanceandinductanceandtheshunt conductanceand capacitance.The characteristicbehaviorof the transmissionlineis primarily determinedby the series inductanceand shuntcapacitance.Arepresentationof the

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transmission line.togetherwiththe sending-endand receiving-end generators.IS

shownin Figure 1.4 [2).

~ r

V,

I

LinelnduClance Line Capacrtarc e

Figure1.4Lumpedelement representationof a lossless transmissionline [2J Thepower transmittedthroughthetransmissionlineis givenby p= V,V,Sinb'

Z"SinfJ

Where

V,is the magnitudeofthese nding -e nd voltage V,isthemagnitudeofthere ceiving- e nd voltage

b'isth e phaseanglebetwee nV.and V,(tra nsmis sion or load angle). Zois Ihe surgeorcharacteristicimpeda nceofthetransmiss ion gr ve n by

1.2 I andcare the series inductance perunitlength andshuntcapacitance per unitlength respectively.

e

is theelectricallengthof the line expressedin radiansby

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e= ¥ a= Ila,Aisthe wavele ngth,

fJ

isthenumber of completewaves perunitline

length and ais the lengthoftheline.

1.3

Itis wellknown thatthe voltagealongthe transmissionlineremainsconstantonlyat surge impedance or natural loading whenthe transmitted powerisP,,=V,,2/4, where V"isthenominalorrated voltageof the line.However, transmissionlinesrarelyhave surge impedance loading. Atlighter loads the receivingend voltage increasesand for heavierloads it decreases[2].

Consider a simple two machine system,which has an inductive transmissionlinewith zero shuntcapacitance,as shown inFigure1.5 with the correspondingpowertransmissionvs.angle characteristicas shownin Figure1.6.

XI2 I XI2

fLTTI

Figure1.5Twomachine powersystemwithinductive line [2]

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·,11

• p.. ,

8(radian)

Figure 1.6Power transmissionvs.angle characteristic The relationshipsbetween real power P,reactive power Q andangle;; are shown in Figure1.6.At a constantvoltage(V,=V,.=V)and fixed transmission system (Xe constant) the transmittedpowerisa functionof englee.Thereal power, P, cannot becontrolled without changingthe reactivepower demand on the sendingand receivingends[2).

1.2.2Steady State Limit of Power Transmission

The maximumpowerP.,.,

=X ' v'

transmittable over a line atagiven

transmissionvoltageisdetermined by the linereactanceX andthussets the limitfor steady state powertransmission.In real-time.practical limit for an actual transmission line withresistanceRmaybeimposed bythe /'Rlossthat heats theconductor. Ata high temperature,the characteristicsof theconductor would irreversiblychange(e.g., itcouldgetdeformed with permanentsag).This setsthe thermallimit forthe

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maximumpower that canbetransmitted.Basically.for long lines the reactanceX and for short lines theresistance R limits the power transfer. As theline length increases, the power flow islimitedby steady state stability[4).

1.3 Line Compensation and Power Flow Control

The steady-statepowertransfer canbe increased and thevoltage profile along theline canbeimproved by appropriatereactivecompensation.The main purpose of reactive compensationis to change thenatural characteristics ofthetransmission line to make it more compatible withthe prevailingload demand. Thus, shuntconnected switchedreactors areapplied to minimizelineovervonageunderlightload conditions and shuntconnected fixed or mechanically switchedcapacitorsare used to maintain voltages under heavyloadconditions.In some cases there is need to change the naturally imposed transmissionangleof the line.Inthis case, a phaseshifter canbe used to controlthe angle of the lineindependent ofprevailingoveralltransmission angles.

1.3.1 Shunt Compensation

Considera simpletwo machine(two bus) transmission systemmodel in which a staticvar compensator (SVC) is shunt connectedat the midpointof the transmissionline as shownin Figure1.7.A staticvarcompensatorgeneratesor absorbs shuntreactivepoweratits pointof connectioninthe transmission line.This compensatoris representedby a sinusoidalvoltage source (of the fundamental

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frequency).inphasewiththe midpointvoltage,Vm, withan amplitudeidentical to that ofthe sendingand receiving endvoltages(Vm=V.=V,=V ).

Themidpointcompensatorin effectdividesthetransmission lineinto two independentpans:the firstpart, with animpedance ofXJ2,carries power from sendingend to themidpoint,and the secondsegmentalsowithan Impedance ofXJ2, carries power fromthe midpointto the second receivingend.There is onlyan exchangeof reactive powerwith the transmissionline inthis process.

0.1 Ism Imr 0.1

'"

^~

I ^~ ^'"

Ideal

co m).

^J'

(P=Q

r

J' v, v. 'V

I

Figure!.7Two machinepower systemwithan idealmidpointreactivecompensator 12J

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13

4V' /

Q ' =-X('

r

^C" " ) / p

=

2V'

,in":

I

^P X 2

/~~==-- ~P.=V'Xsin8

/; (radian)

Figure1.8 Powertransmissionvs.anglecharacteristic The relationshipamong real powerP,reactive power Q,andangle Sfo r thiscase of shuntcompensation is plottedin Figure 1.8 where asP,andQpare changed realand reactivepower. Thefigure shows thatthe midpointshuntcompensationcan significantly increase thetransmittable power.The concept of transmissionline segmentationcan be extendedtothe use of multiple compensators located atequal segmentsof transmission line.Theoreticall y,thetransmittablepowerwould double witheach doublingof segmentsfor thesame overall line length.As thenumber of segments are increasedthe voltagevariation alongtheline wo uld rapidlydecrease, approachingtheideal caseof constant voltageprofile.

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Eventually, with a sufficiently large number of line segments, an ideal distributed compensation system could theoretically be establishedwhichwould have the characteristics of surge impedance loading,but would haveno power transmission limitations, and would maintaina flat voltage profileat anyload [2],

1.3.2 Series Compensation

Series capacitive compensation decreases the overalleffective series transmission impedance of the line from the sending-end to the receiving-end.The impedance of the series connected compensating capacitor cancels a portion of the

actualline reactance and thereby the effective transmissionimpedanceis reducedas if the linewasphysically shonened. The maximum powertransfer capabilityof a transmission line may be significantlyincreased by the use of series capacitor banks.

Consider theprevious simple two-machinemodel with a seriescapacitor compensatedline, composed of two identical segments showninFigure1.9.

Xcl2 XJ2 I XJ2 Xcl2 p

r·Tll

Figure1.9 Two machinepower system withseries capacitive compensation [2]

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"

[, (radian)

Figure 1.10 Power trans missionvs.angle characte ristic

The relationshipamongthe real power P, series capacitorreactivepowerQ,c.and compensationk(de gree ofseriescompensation)is shown inFigure1.10.Itcan be observedthatthetransmutablepowerrapidlyincreases withthe degreeofseries compensationk.Similarly.thereactivepower suppliedbytheseriescapacitoralso increasessharplywithk and varieswith anglebina similar manner10theline reactive power.Series capacitors have beenusedto compensa tefor verylong overhead lines.There has been an increasin g recognition of the adv antagesof compensatingshoner, butheavilyloaded,linesbyusing series capacitors.

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1.3.3 Phase Angle Control

Inpower systemoperations,itis possiblethatpowertransmission angle requiredfor theoptimaluse of a transmissionline woul d beincompatible with the properoperationof the overalltransmission system.Suchcaseswould occur.for example,when power betweentwobuses is transmittedoverparallel lines of different electricallength orwhen two buses are internedwhose prevailing angle difference is sufficient to establis ha power flow.Inthese casesa phaseshifterorphase angle regulatoris frequentlyapplied.The conceptisexplainedwiththe two mach inesystem inwhicha phaseshifteris insertedinbetweenthesending-endgenerator(bus) andthe transmissionline, as illustrated inFigurel.l1.The phase shifter can be

v,

...L.. ^X

t t

V..

rt

V, "'\...

I I

Figure1.11Two machinepower systemwith aphaseshifter [2]

considered as a sinusoidalac voltagesource with controllab leamplit udeandphase angle.In otherwords,the sendingendvoltage V,becomesthe sum of generator voltage and thevoltage

v

oprovidedbythephase shifter.The power can be kept at its peakvalueafter angleJexceedsnl2(thepeakpower angle)by controllingthe amplitudeofquadrature voltage V(Tso that the effective phase angle betweenthe sendingand receivingend voltages staysatfi/2.This way,thepo we r transmitted

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through thetransmission line maybeincreased significantlyeventhough the phase shifterdoesnol increase the steadystate powertransmissionlimit.The relationship between realpowerPand anglese5andais shown plotted in Figure 1.12.It is seen that,althoughIhe phase shifter doesnotincreasethe Iransminablepowerof the uncompen satedline,yet it makes itpossibletokeep the powerat itsmaximumvalue at anyangler5in therangeIf12c,')'<tr/2+aby,ineffect,shiftingthe P versuse5 curve10the right.Itshould be noted that thePversuse5curve can alsobeshiftedto the leftbyinsertingthe voltage of the phase shifterwithan oppositepolarity[2J.

Figure 1.12Power transmission'IS.anglecharacteristic

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IS

1.4 Flexible A.CTransmissionSystems

Basic transmission problems that have caused inefficie nt use of transmissionand other assets have motivatedpowersyste m engineerstoconsider powerelec tro nic s based system for traditionalcompensation techniques

The Flexible A.C Transmission System (FACTS) technology developed to overcome the problemsin the power systemsduetoincrease in demand and supply of power and restrictionon constructionof transmissionlines because of necessary right of ways.The main objectives of FACTStechnology are

To increasethe powertransfer capabilityof transmissionsystems.

To keep power flow over designated routes.

Thefirst objective impliesthat power flow ina givenline shouldbeable to beincreasedup to the thermal limitby forcing the necessary current through theseries lineimpedance.FACTS technologyhelps in operating the transmissionlines attheir thermal limits which results intransmitting more power. This motivedoesnot mean to saythat thelines wouldnormally be operated at theirthermal limit loading (the transmission losses would beunacceptable),butthis option wouldbeavailable,if needed to handle severesystem contingenc ies.Howe ver, by providing thenecessary rotationaland voltage stability usingFACTS controllers,instead oflarge steady-state margins,the nonnalpower transferoverthe transmissionlinesis expected to increase significantly(5).

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1.4.1FACTS Controllers

The generation ofFACTS controllers hasincorpo ratedtwodifferent approaches,both resultingin a comprehensivegroupof controllers able to address targeted transmissionproblems.The first group of controllers employs reactive impedances or a tap changing transfo rmerwiththyristor switches as controlled elements;the second group uses self-cc mmutared converters as controlledvoltage sources.Inthe next section the two groups of controllers are discussedin detail.

1.4.2 ThyristorControlledFACTS Controllers

The thyristorcontrolledFACTS controllers,static var compensator(SVC), thyristor controlled series capacitor (TCSC) and phase shifter, employconventional thyristorsincircuitarrangeme ntsthatare operatedbysophisticated controls. All these controllers govern one of the parameters controlli ngthe power no winthe transmission system,the SVC for voltage,TCSC for transmission lineimpedanceand phase shifterfor transmissionphaseangle. All of these have a commoncharacteristic that(he necessary reactivepower requiredfor thelineisgenerated or absorbed by the traditionalcapacitor or reactor banks andthe thyristorswitchesareused only for the combinedreactive impedancethese bankspresent tothe ac system. The basic operatingprinciplesand characteristicsof these conventionalFACTS controllersare summarized below.

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1.4.3 StaticVAR Compensator

A shunt-connectedstaticvar compensatoremploys thyristor-switched capacitors(TSC's ) and thyristor-controlled reactors (TC R's).With proper coordination of the thyristor capac itorswitchingand reactorcontrol, the reactivepoweroutput can bevariedcontinuously between the capacitive andinductive ratings ofthe equipment.

Thecompensatorisnormally operated to regulate thevoltage at the bus.The maximum obtainable capacitivecurrent decreaseslinearlywith system voltagesince SVC becomesafixedcapacitor when themaximumcapacitiveoutputis reached.

Therefore,the voltage supportcapabilityof theco nventio nal thyristo r-controlledstatic varco mpensator rapidlydeteriorates withdecreasingsyste mvoltage.In addition10 voltage support.SVCs are also employed for transientand dynamic stability.One of the configurationusinga fixedcapacitor withathyristorcontrolled reactoris shownin Figure1.13. Static var compensatorsare extensive lyemployed in electricutility systems.

' .

{]2i

Figure 1.13 Fixedcapacitor-Thyristor controlledreactor type SVC

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"

1.4.4 ThyristorControlled SeriesCapacitor

The basicTCSC module comprisesof series capacitorinparallel with a TCR.A metal oxidevaristor,essentiallya resistor is connectedacrossthe series capacitor to prevent the occurrence of high capacitorover-voltages.The degree of series compensation inthecapacitiveoperatingregionis increasedbyincreasingthe thyristorconductionperiod andthereby the currentinthe TCR.The principleof variableseries compensationistosimply increasethe fundame nta l freque ncyacross a fixed capacitorin a series compensated line.In a thyristorswitchedcapacitor(TSC) scheme,the amountofseries compensationis controlledbyincreasing or decreasing thenumber of capacitor banksin series. TCSC configuratio n usesthyristor-controlle d reactors (TCRs)in parallelwith segmentsofacapacitoras showninFigure1.14.The TCSC can becontinuously controlledinthe capacitiveorinductivezone.The impedanceof thetrans mission line can be adj ustedusing TCSC 10 meetthe performance requirements.

Figure1.14ThyristorControlled SeriesCompensatorScheme

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1.4.5 Phase Shiller

The principlesfor usinga phaseshiftingtransformerwitha thyristo rtap- changerare well establishe d. Mostl y phase shifterswith amec hanica ltap-changer provide quadraturevoltage injection.A thyristorcontrolled phase shiftingtransformer consistsof a shuntconnectedexcitation transformer (IT), a series insertion transformer andathyristor switch arrangeme ntconnectinga selectedcombinat ion of lap voltages10 thesecondaryofthe insertiontransformers.The phase anglebetween thevoltage injected by the phase shifterand thelinecurrentis arbitrary ,determined by the parametersof the overall system.The thyris torcontrolle d phase-shifting transfo rmercouldbeusedto regulate thctransmissionangle tomaintainbalanced powerflowin multipletransmissionpaths. In general,the phaseshifterexchangesreal andreactive power withtheac system through the series insertiontransformer.A thyristor contro lledphaseshifting arrangementis shown inFigure1.15.

v,

Figure1.15 Sche matic diagramof aphase shifter 12]

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1.4.6 Converter Based FACTS Controllers

The secondgroupof FACTScontrollersemploysself commutatedvoltage source converters torealize rapidlycontrollable, static synchronousac voltageor currentsources.This approach includingthyristor switched capacitors and thyristor controlled reactors generallyprovidesbetterperformance characteristicsand are applicablefor transmissionvoltage.lineimpedance.andanglecontrolas compared to conventionalcompensation methods. It hastheunique potential10 exchangereal power directlywith the ae system in addition10 provide independentlycontrollabl e reactive power compensation,therebygivinga powerful optionforpower flow control.

Someof therecently proposedFACTS controllers employingswitching converter based synchronous voltage sources are, the Static Synchronous Compensator(STATCOM ),the StaticSynchronousSeries Compensator(SSSC),the unifiedPowernow controller(UPFC)

1.4.7 Static Synchronous Compensator (STA TCOM)

The basic operatingbehaviorof STATCOMis similar to asynchronous condenser.Ifthe synchronousvoltage source(SVS)is used forreactive shunt compensation likea staticvarcompensator; thesource ofde energy canbereplaced bya smallde capacitoras shown in Figure 1.16.Thesteadystate powerexchanges between the SVS andtheacsystemisonly reactive.

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The convertercanchargethe capacitorto therequired voltagelevelwhen SVS isused for reactivepower generation. The converterabsorbsa smallamountofreal powerfromthe ac system to replenishits internallosses and keep thecapacitor voltageatthedesired level. The abilityof the STATCOMtoproduce fullcapacitive output currentat low systemvoltagealsomakesit highlyeffective in improvingthe transientstability. Thecontrol mechanism canbeusedto increaseordecrease the capacitorvoltage and therebythe amplitudeof theoutput voltageof the convener,for the purpose of controlling the var generationorabsorption.

FigureL16Static SynchronousCompensator[I]

1.4.8 Static Synchronous Series Compensator(SSSC)

Avoltagesourceinvertercould beused inserieswiththetransmissionline.

Thisdeviceis calledstaticsynchronouscompensatoras showninFigure1.17. An SSSCis capable ofinterchange of active andreactivepowerwiththesystem.The

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injected voltage could becontrolled in magnitudeandphaseif sufficient energy source is provided .The SSSC behaves like a controllableseries capacitorand a co ntrollab le series reactor.The basic difference isthatthe voltage injectedby the SSSC is not relatedto thelinecurrentandcan be independentlycontrolled.

Figure1.17 StaticSynchrono usSeries Compensator[2]

1.4.9 Unified Power Flow Controller (UPFC)

Theunified power flow controllerutilizesthesynchronousvoltagesource (SVS)concept forprovidinga uniquely comprehensive capabilityfor transmission systemcontrol.Itcan providethe functionalcapability of indepe ndentlycontr olling both the real andreactivepower flow of the line.

Y,

Figure1.18Two machinesyste m withthe Unified Po werFlowController[2).

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As shown in Figure1.18, UPFC is represented by a controllable voltage sourcein serieswiththeline.The voltageinjectedby the UPFCin serieswith the line is Vpq(O:::Vpq5V1"1"'.".) atanglep(0:::p:::3600).Theline current flows through the series voltagesource,Vpqandresultin exchangeof real andreactivepower. This impliesthat voltagesource Vpqgenerally exchanges bothrealandreactive power with the transmissionline.Aconverter-based SVS can internallygenerateor absorb the reactive power, but the realpower it exchangeswith,mustbesupplied10,orabsorbed from itsdeterminals.The available sourceof power is thesending-endgenerator. The shuntconverter is assumedtobeoperated at unitypowerfactor, asits main functionis to transferthe real powerdemandof theseries converter to the sendingendgenerator.

Itis reasonable tostipulatethat the sending-endgeneratormust providethe SVS exchanges with the transmission line to accomplish the desiredflowof power.A possiblepracticalimplementationofthis couplingis a back-to-backconverter arrangement,in which the shunt series connectedconverter, provides the realpower (fromthe sending-endbus) the series-cormectedconverter exchangeswiththeline.

Thebasicblock diagramofthe Unified Power flow controlleras showninFigure 1.19.

Figure1.19 Unified PowerFlow Controller

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1.5

Summary

Thischapter has focused onthe basicsof the AC power transmissionto provide anecessary backgroundforunderstandingthe power electronicsbased devices. Beforetheadventof FACTS technology,thetransmission capabilit yof transmissionlines wasincreasedbytraditionallineco mpensatio ntechniques.The mainobjectiveof FACTS isto increase theuseable transmission capacityoflines and control theflow ofpower.The arrangement ofvoltagesource convenersin FACTS devices can provide individualvoltage,impedance and angle regulation or, alternativelyrealand reactive power flow controland thuscanadapt to short term contingenciesand future systemmodifications.Theinstallation ofFACTScontrollers particularlytheconvenertypes with multiple power flow control capabi lityhas revclutic nalize d thefield of powertransmissio nanddistributio n.Economic evaluation andpracticalconsiderationssuggest that FACTSdevices havesignificantcapabilityto enhancetheoperationof a power systemby overcomingsomeofthe traditional limits.

1.6

Objective and Organization of the thes is

Theobjecti ve ofthis thesisis to developandverifya modelfo rFACTS device used incontrollingthe power flow inenergysystems.The modelis intendedto besimplein realization,easy toimplement and applicable to differentpowersystem configurations.Inthisthesis,the termsFACTSControllersand FACfSdevices are uscdi nterchangeably.

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Chapter2 ofthis thesispresents thefundamentalsofUPFC,principleofoperationof UPFC,variousconverter topologiesand controlmodes ofUPFC. A detailedanalysis of convene rswitching technique has beendemonstrated withsimulationresults.

Chapter3 discusses theinjectionmodel ofUPFC.The UPFC modelused inthe simulation is derivedand explained.TheUPFC model has beenincluded in aNewton- Raphson load flowalgorithm. Chapter 4illustrates the applicationofUPFCfor powerflow controL The UPFChas been used to reduceoverloadsin a transmission line and improvethe voltage atthe buses. Varioustest systemshave been used to showthe effectivenessof theUPFC.Chapter5 discussesthearea of available transmissioncapabilityinelectric power systems.Anumberof methods to calculate transfer capability have beendiscussed.Theeffect of UPFCis studied and demonstratedwithdifferent power systems.The resultsclearly illustrate the effectivenessof UPFCin enhancingtheAvailableTransferCapabilityof power systems.In Chapter 7,the summaryofthethesis highlightingthecontributionofthe researchandsuggestions forfutureworkare outlined.

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"

Chapter 2 The Unified Power Flow Controller

2.1 Introduction

Thepower transmittedover a transmissionline isafunctionoftheline impedance,the magnitudeofsending-endvoltage, receiving-e ndvoltageandthe phase angle betweenthevoltages.As presentedinchapterI, traditionaltechniquesof reactiveline compensationandsteplike voltage adjustment arc generallyused to alter theseparameters to achieve optima lpower transm issioncontrol.

Theunified power flow controller(UPFC)is ame mber ofthe groupofFACTS devices whichutilizessynchronous voltage sourceconcept for provid inga comprehensivecapab ilityfortransmissionsystemcontrol. Withi nthestructure of traditional power transmissio nconcepts,theUPFCis able to controlsimu ltaneously or selectively allthe parametersaffecting powerno win transmissionline.Itcan also

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)0

providetheuniq ue capabilityof independentlycontrollingboththe realand reactive pO'oo,'crflow inthe transmission line. Thesebasic featuresmakethe unifiedpower flow controllerthe most powerfuldevice presently availablefortransmission system control.This chapterpresentsoperatingcharacteristics andfeatures of theUPFC.The various features have beensupportedbysimulationresults.

2.2 Operating principlesandcharacteristicsof UPFC

The unified powerflow controlleremploys Iwo switchingconverters, which are considered as voltagesourceinvertersusinggate tum off(GIO)thyristor switches,asillustrated in Figure2.1.These converters,labeled"ConverterI"and

"Converter2" are operated froma common de linkprovidedbyade storagecapacitor.

This arrangement functio nsas an idealac to ac powerconverterinwhich the real power can freely flowineither directionbetweenthe acterminalsof thetwo convertersandeach convertercanindependently gene ratereactive poweratitsownac outputterminal.

Converter outputpro videsthe main functionof theUPFC byinjecting an ac voltage Vpq (O::;Vpq:5Vpqm,u)at phase angle p(<>::: p:.::J60")at thepower frequenc y,in series with thelinewith aninsertionseriestransformer. This injectedvoltagecan be consideredessentiallyas an acsynchro no us voltagesource.The transm issionline current flowsthrough the voltage source resultingin real andreactivepowe r exchange between it and the ac system.Thereal powerexchangedat the ac termin al(i.e.,atthe terminalof the seriesinsert io n transformer)is convertedbythe converterinto de

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31

power whichappearsatthede linkaspositive or negativereal power demand. The reactive powerexchanged at the aeterminal is generated internallybytheconverter.

sh unt side seri es

~ 1~

Figure 2.1:Basiccircuitarrangementof theUPFC serie s sid e

ICD

Thebasic functionof convener1 is to supply or absorb the real power demandedbyconvener2 at the common delink.This de linkpowe r is convertedback toacand coupledto the transmiss ionline via a shuntconnected transformer.

Converter1 canalsogenerate or absorbcontrollablereactivepo wer,ifitis required andtherebyitcan provide independentshunt reactive co mpe nsatio nfortheline.There is a closedpath for the real powerexchangedbythe actionof series voltageinjection through converters1and2 back to the line.The reactive powerexchangedis supplied or absorbed locally by convener1and 2 back10 theline.

ConvenerI can beoperatedat a unity power factoror can exchangereactive power with thelineinde pendent of any reactive power exchange by convener 2.

Hence thereis noreactive power flow throughthe UPFC.

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The functionalcapabilityof the unified power flow controllerfrom the traditional transmissio nconceptis based onshuntcompensation,series compensationandphase shifting.TheUPFC can fulfillall thesefunctionsandtherebymeet multiple control objectives by addingthe injected voltage Vpqwithappropriateamplitudeand phase angle, to the terminal voltageV.Using phasor representa tion,the basicUPFCcontrol functions areillustrated in Figure 2.2.

(bj (c) (dj

Figure 2.2 Phasor diagramshowingcontrolcapabilityofUPFC[II

Figure2.2(a)shows termina l voltage regulationsimilar to thatwhich can be obtainedwith a transfonncr tap-changerhavinginfinitelysmall steps.Vpq=tN is injectedin-phase (or anti-phase)with V

Figure 2.2(b)showsseriesreactive compensationwhereVpq=V_cis injectedin quadraturewithlinecurrentI.The seriesvoltageinjectedinthetransmission line can

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bevariedinproportion to the linecurrent toimitatethecompensationobtained witha seriescapacrror orreactor.

Figure 2.2(c) shows phase angleregulation.Vpq=V"isinjected at an angle with respectto V,that achievesthedesiredphaseshiftwithout any change in magnitude.Thismodecanfunctionasa phaseangleregulator.

Multifunction power flowcontrol isexecuted by simultaneous terminal voltage regulation,seriescapacitivecompensation and phaseangle as shown in Figure 2.2(d) where Vpq=,iV+Vo+V",Allthe parameterscontro lling the flowofpowerin thetransmission linecanbecontrolled simultaneously.The powerfulfeaturesof the UPFCcanbeusedtomaintain real and reactivepower flow inthe line at desired levels.The UPFCcontrolsthe magnitude and angularposition ofthe injected voltage soasto maintainor vary therealand reactivepowerflowin theline to satisfyload demand andsystem operatingconditions

2.3 Realand ReactivePowerControl using UPFC

Asimpletwo machinesystemwith sending-endvoltage V•.receiving-end voltageV"and lineimpedance X is shown in Figure2.3 (a). Figure 2.3 (b) shows the systemvoltagesintheform of a phasor diagramwith transmissionangles/) and

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(b)

Q"Q,

(radian)

(e )

Figure 2.3 (a)A simpletwo machine system(b)Relatedvoltage phasor,(c)Real and reactivepowervers us transmissionangle.

In Figure 2.3 (c) the transmitted power P (P""{V^2/X}sin 0)andthereactivepower Q=Q.=QR(Q=(V2/XI{Lcosfi ]')supplied atthe endsoftheline aresho wn plottedas afunctionof the angle0(i.e.,O:::O~").Theelementarysystem ofFigure2.3(a)has been used asabuilding block to explainthecapabilityof theIJPF Cto controlthereal powerPandreactive powerQ.andQII..at the sending-end and therecei ving end of line respectively.

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35

Figure 2.4showsthepower systemofFigure 2.3(a)included withIIUPFC.

TheUPFC isrepresented byacontrollablevoltagesource inseries with the line which,as explained in the previoussection,can generateorabsorbreactive power from the sending-endgenerator. Thevoltageinj ected bytheUPFCin serieswiththe

Figure 2.4Two machinesystem withthe Unified PowerFlowContro ller [2].

lineisrepresented by VP'lhaving magnitudeVpq(O:';;:Vpq$Vpq"",.)and anglep (O:';;:p$3600).Theline currentflows through the seriesvoltagesource,Vpq'and results in bothreal and reactive powerexchange withtheline.Torepresent theUPFC properly,the series voltagesourceis designed to generate only the reactive powerQpq itexchanges withtheline.Thus therealpowerPpqit exchangeswiththelineis assumedto betransferred to thesendingendgeneratoras if aperfectexchan geforrea l powerflowbetween it and the sendingendgenerator exists.This is compatible with the UPFC structureinwhichthedc link betweenthetwo voltagesource converters maintainsabi-directionalcoupling forreal powerbetween the injectedseriesvoltage and thesending bus.In Figure 2.4 the shunt reactive compensationcapabilityof the

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UPFCis not utilized. TheUPFC shuntinverteris assumedtooperateatunitypower factor.Its mainfunctionis to trans ferthe rea l powerdemandof the series inverter to thesendingendgenera tor .Theoperating principle ofthe UPFCis explainedwiththe help of asimple two bus power system[6].

2.3.1 UPFCinatwo-buspower system

--+Q, Figure2.5 Conceptualrepresentatio nofUPFC ina two-Bus Power System

The power flow controlcapability of the UPFCcan be illustratedbythe real and reactivepowertransmissioncharacteristics ofthe simpletwomachine system shownin Figure2.5. The system consists of sendingand receivingend voltage sources. A voltageof appropriatemagnitudeVI"landphaseanglepis addedtothe (Sending-end)terminal voltageV,.

Thesystem voltagesinphasorfonnaredefinedbelow:

V. =V e~JI1 2 V,=V eil/l VPo'=V""eiP

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37

The com plex powerflowis givenby V,Ir'andcanbewritten as P.+jQ" where ..

means theconjugate.Currentflowing through thelineis given by Ir =V.~+vP't-V,

jX

Substituting the value ofLinV,I;

Vs +Vpq-v, . P,+j Qr=V,C- -jX- - l

Taking V,insidetheequation

. ( v :v, + V ;V,., -V:VrJ'

p,+JQ , = jX

Substituting the value of voltage

( y ,

^"^+VV

'(~" L v,]

Pr+jQ,=

e 7 / .

(2.1)

(2.2)

(2.3)

equation (2.3) gives

(2.4 )

Expressi ngei°as coso+js inoandexpandingequation2.4gives

Takingtheconjugateof equation2.5gives

[

Ii . . .

s ]

V¹cos(<5)+VVI"lcos(-+P)_V.²-)(V²sm(o)+ W""sm(-+

p»

Pr+jQ.= 2 2

-JX

(2.5 )

(2.6)

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Separatingthereal andimaginaryparts of equatio n2.6thefollowing equation.is obtained.

P, andQ. canbe separatelywrittenas J p v¹sin (o ) + W",sin(2"+p)

,- X X ^(2.8)

(2.9)

(2.10)

(2.11)

are the real andreactivepowers characterizing the power transmission of the uncompensated systemat a given angle

o.

Since: anglep isfreelyvariable between0 and21: atany transmissionangle 0(~).with these capabilitiesof theUPFC explainedabove, theconventionaltermsof series compe nsation.phase shifti ng.etc.

become irrele vant;theUPFC simplycontrolsthe magnitud e: andangularpositionof injec tedvoltage.

Itcanbeobserved fro m Figure2.4 thatthetransmissionlinc sees V,+VP"Iasthe effectivesending endvoltage.TheUPFCaffectsthe vo ltage (bothitsmagnitude and

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)9

angle) acrossthetransmissionline and thusitis able to control, by varyingthe magnitu deandangleafVP'!'the transmissi on real power aswell as thereactive power demandof theline atany giventransmissionanglebetweenthe sending-end and receiving-endvoltages .

Figure2.6 showsthe controllablearea of realpowerPandreactive power dem andQattainedwith the UPFCfor the voltageinject edat differen t angles8=0", 0=30",0=60^% =90".The centers ofcontrol regionsaredefinedbytheP.,(O).QOf(S) coordinates.Figure2.6(a)illustra testhecase whenthe transm ission(ph ase)angleis zero(0=0).As Vpq=Oiszero, P,Q,andQ.are allzero. The circleistheloc us of all values of realpower Pand reactive powerQfobtainedfor a fixedvalue of injected voltagephasor rotatedatfull revolutionof (o-;;p::53600).The area withinthis circle definesall PandQ,values obtainable bycontrollingthe magnitudeVpqma,and anglep ofphasorVpq•

The circle in the{P.

Q,l

planedefinesallPandQ,valuesattainab lewiththe UPFCofa given rating. Asanexample,theUPFCwith thevoltageratingof Vpqp.u isable to establishVpqp.u.powerflowineitherdirection,withou t im posing any reactivepower demandoneitherthe sending-endor the receiving-endgenerator.The UPFC canalsoforcethe system at one endtosupply reactive power for,or absorbthat from,thesystemattheotherendf7j.

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Figure 2.6 Control region of the attainablereal power P and receiving-endreactive power demandQ,with a UPFC-controlledtransmissionline at0=0"(a)

0=30"(b) 0=60⁰(c)0=90"(d)(I]

2.4 ConverterTopologies

Converters areused to convert de power intoac power at desiredoutput voltage andfrequency.The force comrnuratedinverters ean be classifiedintocurrent source convertersand voltage source converters. The two basic categoriesof self commutating convertersare explainedbelow.

The current sourceconverter converts power betweenanadjustablecurrent source and a single or threephase load. Since the inputpoweris'usuallysuppliedat a constant voltage,the convertersystem consists ofthetwostages.

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Figure2.7 showsthat the currentsourcereceives power from afixed voltageac orde bus and suppliesan adjustab ledc currentatthe output terminal s.This stage employsa controlledbridgerectifie ror ade chopper.The curren tsource converteris connectedwiththe "current source".Itconve rts thede current at the inputto an ac currentat the outputterminals. The de currentalways flowsin onedirection and power flowwould reverse with change in direction of dc curren t. The amplitudeof the ac currentcan be controlle donly indirectlyby controllingthe magnitudeofthe de inputcurrentproducedby the current source.

Figure2.7showsthe converterbox witha simplesymbo l because the current source conve rtermay be basedon diode s, conventio nal thyristo rs orthe tum off de vices.

de Power

Figure 2.7CurrentSourced Converter] I]

Convertersfedby a de sourcewithsmallinternal impedanceare calledvoltage sourced converters. Con verters employingthyris torsconnected inbridge configur ation are calledbrid ge conveners.

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~ ^{I r ~:::::}

d'POW" ~ " U

^____

_.

^ReactIVe

Puwer

Figure 2.8 Voltage Sourced Converter[1]

Figure 2,8 showsthe basicoperationofa voltage sourced converter. The structureof theconverter shows a box with a gatetum-off thyristor and diode.Theinput side of theconvener hassingle polaritysupported bya capacitor.The sizeof thecapacitoris verylarge toaccommodate currents that accompany the switchingsequenceofthe convener .Thevoltage of thecapacitoriskept constant. The de currentcan flow in either directionanditcan exchangede power withthe connecteddesystemineither direction. Thegenerated aevoltage connectstothe aesystemthroughan inductor.

The voltage source is interlaced with thetransmission linethroughan insertion transformerinaUPFC. The main advantageisits capabilitytotransfer powerin either direction. TheUPFC employs two three phase voltage source convene rs,one connected in seriesandotherin shuntwith the line. Withthese conveners,the ac outp utvoltage magnitude ,phase angleand the frequency can be controlled,by varying the width of thevoltagepulses, and/or the am plitudeof the dc busvoltage.Another approachistohavemultiplepulse sper halfcycle, and then vary the widthof the pulsesto varythe amplitu deof theac voltage.Themainreasonfor doingsois tobe able to vary the ac output voltageandto reducethelow orderharmonic s [8}.

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2.5 Control Circuit

Controlcircuit constitutesamajorpan ofthe completeUPFC device.The controlcircuitisdivided functionallyintointernal (convener)controlandexternal control.The internalcontroloperatesthe twoconverter so as toproducethe desired injectedvoltage. In thissectiona briefintroduction is givento the techniques a voltage sourceconverteremploys to generate variableacoutputvoltage.The technique isbased on pulsewidthmodulation. Thecapabilityofinjecting voltage with specified magnitudeandanglein series with the transmissionline lieswiththe UPFC. Thevoltageis generatedfromthe voltage sourced converter usingsinusoidal pulse width modulation(SPWM)technique,a controlwithinthe inverter and isalso knownasduty-cycle regulation.This methodof regulationof voltage employs variationof the conductiontimepercycle to alterthe rmsoutput voltageofthe inverter.Itisrequired to varythemagnitude of the ac output voltage withouthaving to changethe magnitudeof thedevoltage.Thethreephaseconverter canaccomplishthe desired criteria. With theseconvertersthe acoutputvoltagecan be controlledby varyingthe width of the voltage pulses,and/or byvarying theamplitudeof debus voltage.

The sinusoidal pulse width modulationhas been explained with reference tocontrol requirements of the circuit.In this method,thefiring instantsof the GTO'sof the convenersare decidedbythe interactionsof rectified sinusoidvoltageofamplitude A2witha triangular wave of amplitudeAI.Both thesewaveformsare synchronized withrespectto themainssupply. The outputvoltageis variedby changingthe

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amplitudeofthe rectifiedsinusoidortheratioAllAIas itwould change thewidthof the pulse.The ratio A2IAIis calledthe modulation index.The triangularwavecanbe timedto have eitherpeakor zero withthe zero of thesinusoid [9J.As an example,the followingsection explainshowavoltage sourcedconvertercanbeused to generate and controlthe voltage which is injectedinto the transmissionline through aninsertion transformer.

2.5.1Voltage SourceCon verte rUsingSPWM

To demonstrate the conceptof voltagesourced converter,thecircuitin Figure 2.9has beensimulatedusingPSCADIEMTDC simulationsoftware[10].The basic building block to simulatea voltagesourced converteristhe generation offiring pulses forthe GTOs connectedin the circuit.Figure2.9showsthe system used for generatingthe pulse width modulated signal forthe voltagesourced converter.The threesinusoidalsignals separatedby120⁰aregeneratedusingbuilt-inbasicPSCAD blocksandare comparedwitha triangularwaveusing a triangularwavegenerator.

Both the signalsare comparedusing a comparator.

The output of the comparatorwithadesired modulationindex is a Pulse widthmodulated signalfed10 the GTO'sof the three phase voltagesource converter.Figure2.10showsthe completesystemfor generatingthe variablevoltage through avoltage sourcedconverter.

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Figure 2.9: Circuit arrangement for SPWM Figure 2.10consistsof a rectifier,a d.c source and a voltage sourced converter.In general.the input voltage from athree phase source operatingat 60Hzfrequency is fed to the three phase fullwave rectifier; it ismadeup of diodesconnectedin parallel and givessill pulse ripples at the output voltage.The diodes are numberedin orderof conductionsequences, as 12,23, 34,4S,S6 and 61.The pairof diodes which are connected between the pair ofsupplylines havingthe highest value ofline-to-line voltage would conduct.The output of the rectifieris fed co the inverter.The voltage source converter is made up of an asymmetrictum-off device such as

oro

witha paralleldiodes connected inreverse.Sometum-off devices, suchas the IOBT'sand IOCT's,may have a parallelreverse diode built inas a part of completeintegrated device for suitable voltage source converters as shownin Figure 2.10.Ho wever,for highpower conveners,provision of separate diodesisad vantageo us.In reallime, there wouldbe several tum off device

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=

..

,

s ~ ,.-

"~ ' -"

⁵ ³

^I~

,

^s ^z ²^~

^.. ^. ^.. ~ II!

Figure 2.10 Circuitarrangementfor generationof injected vo ltage.

unitsin seriesfor high voltage applications..The gatingsignalsof threephase inverters sho uldhe advanced ordelayedby120⁰withrespecttoeach otherin order10 obtainthree-phase balanced voltages[11).EachGTDco nd uctsfor180⁰and three ororemain onatanyinstantof time.Athree-phase inverter maybe considered as threesingle-phaseinvertersandtheoutputof eachsinglephase inverter is shiftedby 120°.Inverteroutput voltagecanbecontrolledbyintroducing ac regulatorbetweenthe inverterandthe load. This method has the drawbackthat itgives a high harmonic content. Hence,the outputvoltage is controlledbyvaryingthe pulsewidth ofthe gatingsignals.

Animportantadva ntageof this schemeisthatatlow output voltages,the relative harmonic content does notincrease.The outputof thevoltage sourced converter has beenshowninFigure 2.11.Thefirstplotin Figure 2.11istheinverter outputcurrent

r..

in phaseA.thesecondplot showsthe inverteroutput voltageofphase a-b.E.b'andthe third plotshows thephasevoltageE. of the inverter.Ifthe dcinput

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~0

^..

1~ ~

~005

1

_, ₀

~-O.05

~

!

l _o.IL- - L---'-

~

"

~ ~ ~

u

~ ~ g g

U

ti"..(Secord}

~WIIWW

~0 5~

-b.

^Ll- --'0.1C-2 - 0c'-.I-. ---,O-'-.16,----,O.LI8---,OL.2--'"O.22c:--0--'2--,.- 0c'-2-:-6---'0-'-.28 0.3 lime(Secorxl)

Q12 Q14 Q16 [18 ~ ~ ~ Q~ ~ ~

time (Secooo) Figure 2.11 Plot of inverteroutputwaveforms

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48

voltage to theinverter isvariedto compensateforsource voltagefluctuations,the invertercanbedesignedforthe maximumvalueof outputvoltagedesired[12].

2.5.2Control of Shunt and Series Converter

Thecircuit arrangementoftwo coupledconven ers offers series voltage injection along with independently controllable reactive power exchange and facilitatesseveralcontrolmodes for the UPFC.Reactive shun!compensationand free control ofseriesvoltageinjectionare the optionsto the approach selectedforpower flow controL

Theshuntinverterisoperatedin sucha way so as 10 drawcontrolled current from the acbus.Thecurrent referenceis chosentosatisfy theshunt reactivepower referenceand(inUPFC configuration)toprovide any real power neededto balance thereal powerofthe seriesinverter. A small amount of realpowerisalsodrawnto meetthepowerlossesof the inverterandmagnetics.The shuntreactivepower reference can be either capacitive or inductive.In var control mode, thesimplevar request ismaintained by the control system regardlessof bus voltage variation.The magnitudeandangleofthe voltage injectedinserieswiththelineiscontrolled bythe

The seriesinverter generatesa voltage vector (across theline-side terminalof theinsertiontransformer)withmagnitudeand phase anglerequestedbythe reference input.In automatic power flowmode, series injected voltage isdetennined automaticallyandcontinuously bya vectorcontrolsystem10 ensurethat the desired

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49

values of real and reactive power (PandQ) (looking in to the transmission line) are maintaineddespite systemchanges. Thebasic functionof seriescompensationis 10 controlthe magnitude oftheinjectedvoltagein proportion to the linecurre nt, so that seriesinsertion emulates reactive impedance whenviewed from the line.The injected voltageis controlled withrespectto the inputbus voltage so thatthe output bus voltage is phase shifted relativeto the input voltagebyan anglespecifiedbythc reference input(13].

2.6 Summary

This chapterhas presented the detailsof UPFC.Comparedto other power flow control devices ,such as STATCOM, TCSCetc,UPFC isunique in uscapability 10 control bothreal andreactive power in a transmissionsystem.Variouscontrol features ofUPFCwithvoltage control techniques have been discussed with supporting sim ulations results.The featuresprovidedbyUPFCcan be effectivelyused 10 control powerflow overlong distancehigh voltagetransmissionlines.

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50

Chapter 3 Modeling of Unified Power Flow Controller for Load Flow Studies

3.1 Introduction

Inthis chapter,a generalmodelis derived for theunifiedpower flow controller. This modelwhichis referred10 as injectionmodel is helpfulin understandingtheimpact of UPFC on a powersystem. Thismodel can easilybe implemented inexisting powersystem analysisprograms,particularlyforloadflow studies.Thischapterpresentstheincorporation of the derived UPFC modelforload flow analysis studies.

Fewmode ls have beendeveloped for the UPFC tostudyitseffect onpower system. With sucha comprehensivecontrolcapability, UPFC has theability10 providerealtimecontrolof powerflows within a power system10 meet predefined operatingtarget.Thismaybevery helpfulinthe continuingderegulationofpowe r

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51

ind ustry.Po wer systems connected with UPFCscan precisely controlthe power exchanged in powertransactions.

Thisapproach of modeling incorporates,a series reactance together with a set ofactiveandreactive powerinjectionsat eachendof theseriesreactance.These powers are expressed as functionoftheterminal,nodalvoltages,andthevoltage of a series sourcewhich represents theUPFC seriesconverter.

3.2 Injection Model of SeriesControllable Device

Figure 3.1shows the equivalentcircuit diagram of a seriescontro llable device(SeD)ora series connectedvoltagesource, which is locatedbetweennodes iandj ina power system.A UPFCinjects avoltageV..inserieswiththe tran smissionline throug ha seriestransforme r.TheactivepowerP",involvedin the series injectionistaken fromthe transmission linethrough ashunttransfo rmer(i.e.

P,h)·

The UPFCgeneratesor absorbs theneeded reactive power(i.e.Q,<andQsIl) locallyby the switchingoperationofits converters,while thereisnotransfer of reactive powerthrough thedclink.InFigure3.1 x,istheeffectivereactance of the UPFCseenfrom the transm issio n line side of theseries transfo rmer.Theshunt converte rmustsupplyor absorb therequiredamountof activepower tothe series converterviatheDC link.Typically thisamountis lessthanthe MVArating ofthe converter whichallows theshuntconverter to provide reactive power compensation

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10 thesending end.Inthis manner the conveners ofthe UPFC may generateor absorbreactive powerinternally.Figure 3.2showsthe vector diagram of a series controllabledevice [14].

v, v

IX,

v,

Figure 3.1: Equivalent circuitdiagramofa SeriesControllableDevice

v

V.

I..

Figure 3.2:Vectordiagram of theSeries ControllableDevice

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53

3.2.1Injectionmodel of UPFC

To obtainaninjectionmodel for aUPFC,theseries panof the UPFCas show n in Figure 3.3is consideredfirst.

v, v,

Figure 3.3 Representation of the seriesconnec ted voltage source The seriesconnected voltage source ismodeledbyan ideal seriesvoltagev,.

which is controllablein magnitudeandphase,that is,V"=rV,e^iY where O:sir :$r....andO$y$2H.V·representsa fictitiousvoltagebehindthe series

(3.1)

The injectionmodelis obtainedas shown inFigure3.4byreplacingthevoltage sourceV••bya currentsourcef"i=-jb,V"in parallelwithx;Note thatb.=i lx, The currentsource[inj correspo ndstoinjectionpowers S, andSjwhicharedefined by

S, =V, (- I.,)"

S, =V ,(I.,)"

(3.2)

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The injectionpowerS,andSjare simplifiedas Si""V,(jb,rVie irr

'=-rb,v,2sin(r )-jrb,V/cos(y)

s,=

v A-

^jb,rVjc^1r^)"

'=rb,VYjsin(8~+y)+jrb,VjVjcos(8~+y)

(3.3)