含风电场的电力系统静态电压稳定性分析翻译复习过程.doc

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1、Good is good, but better carries it.精益求精，善益求善。含风电场的电力系统静态电压稳定性分析翻译-燕山大学本科毕业设计（论文）英文翻译课题名称：含风电场的电力系统静态电压稳定性研究学院（系）：电气工程学院年级专业：09级电力四班学生姓名：张建春指导教师：王珺完成日期：2013.5.8一Theinterductionofsteady-StateCharacteristicsthespeed-torquecharacteristicisquitelineararoundsynchronousspeed.Iftherotorspeedisbelowsynchron

2、ousspeed，theinductionmachineisoperatingasamotorandiftherotorspeedisabovesynchronousspeed，theinductionmachineisrunningasagenerator.themechanicalpowerandthemechanicaltorquearegivenbytheslip,rotorresistanceandtherotorcurrent.Thespeed-torquecharacteristicoftheinductionmachineisquitelineararoundsynchrono

3、usspeed.thetorqueisproportionaltotheinverseoftherotorresistance.Thisimpliesthatitispossibletohaveexternalrotorresistancesconnectedinserieswiththeexistingrotorresistancesofawound-rotorinductionmachine.Bychangingthevalueoftheexternalrotorresistanceitispossibletochangetheslopeofthespeed-torquecharacter

4、istic.Onedisadvantagewiththismethodisthatitisonlypossibletoincreasetheslipusingtheexternalrotorresistances.Thisimpliesthatiftheinductionmachineisrunningasamotor,thenanincreasedrotorresistancewilldecreasetherotorspeed.Ontheotherhand,iftheinductionmachineisrunningasagenerator,theniftherotorresistancei

5、ncreases,therotorspeedwillalsoincrease.Beforesemiconductorswereavailable,onewayofadjustingtheslipwastointroduceexternalrotorresistances.Theexternalrotorresistancewillcauseadditionallossesintherotorcircuit.Whensemiconductorsbecameavailableitwaspossibletorecovertheslipotherwisedissipatedintheexternalr

6、otorresistance.Thus,theslippowercanberecoveredintomechanicalorelectricalenergy;therefore,thismethodiscalledslippowerrecovery.Therotorcurrentmustberectifiedwithadioderectifier.Formotoroperation,therotorcircuitwillseethedioderectifierasaresistanceandthereforethismethodwillworkapproximatelyinthesameway

7、asfortheexternalrotorresistances.Notethatthedioderectifiercannotbeusedingeneratoroperation.Therectifiedcurrentcouldbeconvertedtomechanicalpowerusingadcmotorcoupledtotheshaftoftheinductionmotororfedbackintothegrid.SinceKramerdriverequireanextradcmotoritisofnointerest,whiletheScherbiusdriveisstillinus

8、e.Themainadvantageofthisconfigurationcomparedtotheexternalrotorresistanceisthatthelossesoftheexternalrotorresistancecanberecovered.Ifbothstatorvoltageandfrequencycanbeadjustedbyaninverter,thetorque-speedcharacteristiccanbeeasilychanged.Whenthespeedisincreasedsothatthestatorvoltagereachesmaximumvolta

9、ge,thereisneedforfieldweakening,thestatorvoltageiskeptconstantwhilethefrequencyisstillincreased.二Theinterductionofdoubly-FedInductionMachinesDoubly-fedmachinescanbeusedinvariable-speedconstant-frequencyapplications,suchaswindturbines.Themainadvantageofadoubly-fedmachinecomparedtoasingly-fedforavaria

10、ble-speedsystemisthereducedratingoftheconverterspowerrating.Thereductioninpowerratingisdependentonthespeedrangeofthedrive.Thestandarddoubly-fedinductionmachineisawoundrotorinductionmachineequippedwithsliprings.Thestatorcircuitisconnecteddirectlytothegridwhiletherotorcircuitiscontrolledbyaninvertervi

11、asliprings.Thecascadeddoubly-fedinductionmachineconsistsoftwodoubly-fedinductionmachineswithwoundrotors,thatareconnectedmechanicallythroughtherotorandelectricallythroughtherotorcircuits.Thestatorcircuitofoneofthemachinesisdirectlyconnectedtothegridwhiletheothermachinesstatorisconnectedviaaninvertert

12、othegrid.Sincetherotorvoltagesofbothmachinesareequal,itispossibletocontroltheinductionmachinethatisdirectlyconnectedtothegridwiththeotherinductionmachine.Itisdoubtfulwhetheritispracticaltocombinetwoindividualmachinestoformacascadeddoubly-fedinductionmachine,eventhoughitisthebasicconfigurationofdoubl

13、y-fedinductionmachinearrangement.Duetoalargeamountofwindings,thelossesareexpectedtobehigherthanforastandarddoubly-fedinductionmachineofacomparablerating.三StabilityofaPowerSystemPowersystemstabilityisunderstoodastheabilitytoregainanequilibriumstateafterbeingsubjectedtoaphysicaldisturbance.threequanti

14、tiesareimportantforpowersystemoperation:(i)anglesofnodalvoltages,alsocalledpowerorloadangles;(ii)frequency;and(iii)nodalvoltagemagnitudes.Thesequantitiesareespeciallyimportantfromthepointofviewofdefiningandclassifyingpowersystemstability.Hencepowersystemstabilitycanbedividedinto:(i)rotor(orpower)ang

15、lestability;(ii)frequencystability;and(iii)voltagestability.Aspowersystemsarenonlinear,theirstabilitydependsonboththeinitialconditionsandthesizeofadisturbance.Consequently,angleandvoltagestabilitycanbedividedintosmall-disturbanceandlarge-disturbancestability.Powersystemstabilityismainlyconnectedwith

16、electromechanicalphenomena.However,itisalsoaffectedbyfastelectromagneticphenomenaandslowthermodynamicphenomena.Hence,dependingonthetypeofphenomena,onecanrefertoshort-termstabilityandlong-termstability.四ConnectionsofWindFarmsAlthoughthemajorityofwindturbinesaresituatedonland,thereisagrowingdemandforw

17、indturbinestobeplacedoffshorewithsomelargewindfarmsnowoperational.Thisdoesnotmeanthatoffshoresitesarealwaysbetterthanthoseonshore,assomeonshoresiteshavebetterwindregimesthansitesoffshore.Acommonproblemtoalloffshoreenergyconversionsystemsistheelectricalcableconnectiontotheonshoresubstation.andthisthe

18、nraisesdistanceissuesbecauseallACcableshavehighcapacitanceandthelinechargingcurrentforlongcablerunscanbeveryhigh.whileanumberofindependentcablerunsmaybenecessaryinordertotransmittherequiredpowerfromanoffshorewindfarm.BecauseofthelargecablecapacitanceACcablesarecurrentlylimitedtoadistanceundertheseao

19、fabout100-150kmwiththemaximumratingofthree-coresubmarinecablescurrentlybeingabout200MWat145kV,althoughlargerratingsareunderdevelopment.Generallytheoutputsofanumberofturbinesarecollectedtogetheratanoffshoresubstationforonwardtransmissiontoshore.Oncetheoutputofanumberofturbineshasbeencollected,analter

20、nativetoACtransmissiontoshoreistouseDCtransmission.NewDCtransmissiontechnologyusesIGBTvoltagesourceconvertersatthesendingend(andpossiblyalsoatthereceivingend)allowingtotalcontrolatthesendingend.Forhigherpowers,conventionalDCtechnologyusingGTOscanbeused.Currentlyoffshorewindfarmsaresufficientlycloset

21、oshorethatACcablescanbeused,althoughanumberofcablesmaybenecessarytotransmittherequiredpower.Onepracticalpointtonoteisthatthedistancetoshorealsoincludestheshore-basedcableruntotheshoresubstation.Insomesituationsthiscanbesubstantial.Theproblemsassociatedwithtransferringelectricalpowertoshorefromoffsho

22、rewindfarmsisalsofacedbytidalstreamgeneratorsandwavegenerators.Tidalstreamgeneratorstendtoberelativelyclosetoshore,althoughlayingcablesinthestrongcurrentswheretheseturbinesaresituatedisnotstraightforward.Waveenergyisinitsinfancywiththelargeamountsofresourceavailable.Harnessingthisenergyandtransferri

23、ngittoshoreposesasignificantchallenge.五InfluenceofWindGeneratorsonPowerSystemStabilityThesynchronousgeneratorisstifflyconnectedtothepowersystemandexhibitsaninherentlyoscillatoryresponsetoadisturbancebecauseitspoweroutputisapproximatelyproportionaltothesineoftherotorangle.Forsmallvaluesoftherotorangl

24、e,powerisproportionaltotheangleitselfwhichproducesspring-likeoscillations.Ontheotherhand,squirrel-cage(fixed-speed)inductiongeneratorsarecoupledtothegridlessstifflythansynchronousgenerators.thetorqueofafixedspeedinductiongeneratorisproportionaltothespeeddeviation(slip)henceprovidinginherentdampingof

25、oscillations.Thispositiveinfluenceiscounteractedbythevulnerabilityoffixed-speedinductiongeneratorstosystemfaults.DampingduetovariablespeedDFIGsdependsverymuchontheparticularcontrolstrategyemployed.theDFIGshavegoodcontrolcapabilitiesduetothepossibilityofcontrollingboththemagnitudeandphaseoftheinjecte

26、dvoltage.Thismakesitpossibletodesignapowersystemstabilizerthatimprovesthedampingofpowerswingswithoutdegradingthequalityofvoltagecontrolprovided.Fullyratedconvertersystemseffectivelydecouplethegeneratorfromthegrid,sotheyofferaverygoodpossibilityofimprovingthedampingofpowerswings.Hencethegeneralconclu

27、sionisthatapartialreplacementoftraditionalthermalplantsemployingsynchronousgenerators,whichexhibitarelativepoornaturaldamping,byrenewablegenerators,whichexhibitabetterdamping,willimprovethedampingofelectromechanicalswings.Thiseffectwillbecounterbalancedtosomeextentbythehighlyvariablenatureofrenewabl

28、esourcesthemselves,suchaswind,marineorsolar,buttheirvariabilitymaybeeffectivelymanagedbyeitherusingenergystorageorpartloadingoneoftheturbinesinafarmandusingitssparecapacitytosmoothpoweroscillations.Thenetworkeffectofreplacinglargetraditionalgeneratorsbyrenewableoneswilllargelydependonthesysteminques

29、tion.Recallthatthestabilityofsynchronousgeneratorsdeterioratesiftheyarehighlyloaded,remoteandoperatewithalow,orevenleading,powerfactor.Ifrenewableplantsareconnectedclosertotheloads,thenthetransmissionnetworkswillbelessloaded,whichwillreducereactivepowerconsumptionbythesystemandthevoltageswillrise.Th

30、iseffectcanbecompensatedbyreactivepowerdevices,suchasreactorsorstaticVARcompensators,butthiswouldrequireadditionalinvestment.Ifthatisdeemeduneconomicalandtheremainingsynchronousgeneratorsareusedforreactivepowercompensation,theiroperatingpointswouldmovetowardscapacitiveloading(leadingpowerfactor)soth

31、eirdynamicpropertiesmightdeteriorate.Asthenumberofsynchronousgeneratorsremaininginoperationisreducedduetoincreasedpenetrationofrenewables,theiroverallcompensationcapabilitieswillalsobereduced.Hencetheoveralleffectmightbeadeteriorationofthedynamicpropertiesofthesystem.Ontheotherhand,iftherenewablesou

32、rcesarelocatedfurtherawayfromthemainloadcentres,thenpowertransfersoverthetransmissionnetworkwillincrease.Highertransferswillmeanlargervoltageangledifferencesbetweennetworknodesanddeterioratedsystemdynamicproperties(smallerstabilitymargins).Increasedpenetrationofrenewablesmightalsoaffectfrequencystab

33、ility.Duetoitsconstruction,awindplanthassmallerinertiaandspeedsothatkineticenergystoredinitisreducedbyafactorofapproximately1.5whencomparedwithatraditionalplantofthesamerating.Thereductioninstoredkineticenergywillhaveaneffectonsystemoperationandsecuritybecauseoftheamplitudeoffrequencyvariations.六Vol

34、tageStabilityVoltagestabilityistheabilityofapowersystemtomaintainsteadyacceptablevoltagesatallbusesinthesystemundernormaloperatingconditionsandafterbeingsubjectedtoadisturbance.Voltagestabilitycanbeattainedbysufficientgenerationandtransmissionenergy.Generationandtransmissionunitshavedefinitecapaciti

35、esthatpeculiartothem.Theselimitsshouldnotbeexceededinahealthypowersystem.Voltagestabilityproblemariseswhenthesystemisheavilyloadedthatcausestogobeyondlimitationsofpowersystem.Apowersystementersastateofvoltageinstabilitywhenadisturbance,increaseinloaddemandpowerorchangeinsystemconditioncausesaprogres

36、siveanduncontrollabledeclineinvoltage.Themainfactorcausinginstabilityistheinabilityofthepowersystemtomeetthedemandforreactivepower.Themainreasonforvoltageinstabilityisthelackofsufficientreactivepowerinasystem.Generatorreactivepowerlimitsandreactivepowerrequirementsintransmissionlinesarethemaincauses

37、ofinsufficientreactivepower.Synchronousgeneratorsarethemaindevicesforvoltagecontrolandreactivepowercontrolinpowersystems.Involtagestabilityanalysisactiveandreactivepowercapabilitiesofgeneratorsplayanimportantrole.Theactivepowerlimitsareduetothedesignoftheturbineandtheboiler.Therefore,activepowerlimi

38、tsareconstant.Reactivepowerlimitsofgeneratorsaremorecomplicatethanactivepowerlimits.Therearethreedifferentcausesofreactivepowerlimitsthatare;statorcurrent,over-excitationcurrentandunder-excitationlimits.Thegeneratorfieldcurrentislimitedbyover-excitationlimnerinordetoavoiddamageinfieldwinding.Infact,

39、reactivepowerlimitsarevoltagedependent.However,inloadflowprogramstheyaretakentobeconstantinordertosimplifyanalysis.七AnalysisofvoltagestabilityThemostcommonmethodsusedinvoltagestabilityanalysisarecontinuationpowerflow,pointofcollapse,minimumsingularvalueandoptimizationmethods.Inthisstudy,continuation

40、powerflowmethodiswidelyusedinvoltagestabilityanalysis.Sovoltagestabilitycanbeanalyzedbyusingcontinuationpowerflow.TheJacobianmatrixofpowerflowequationsbecomessingularatthevoltagestabilitylimit.Continuationpowerflowovercomesthisproblem.Continuationpowerflowfindssuccessiveloadflowsolutionsaccordingtoa

41、loadscenario.Itconsistsofpredictionandcorrectionsteps.Fromaknownbasesolution,atangentpredictorisusedsoastoestimatenextsolutionforaspecifiedpatternofloadincrease.ThecorrectorstepthendeterminestheexactsolutionusingNewton-Raphsontechniqueemployedbyaconventionalpowerflow.Afterthatanewpredictionismadefor

42、aspecifiedincreaseinloadbaseduponthenewtangentvector.Thencorrectorstepisapplied.Thisprocessgoesuntilcriticalpointisreached.Thecriticalpointisthepointwherethetangentvectoriszero.Incontinuationloadflow,firstpowerflowequationsarereformulatedbyinsertingaloadparameterintotheseequations.Injectedpowerscanb

43、ewrittenforthei-busofann-bussystemasfollows（1）（2）wherethesubscriptsGandDdenotegenerationandloaddemandrespectivelyontherelatedbus.Inordertosimulatealoadchange,aloadparameterisinsertedintodemandpowersand.(3)andareoriginalloaddemandsoni-buswhereasandaregivenquantitiesofpowerschosentoscaleappropriately.

44、AftersubstitutingnewdemandpowersinEquation(2)toEquation(3),newsetofequationscanberepresentedas:(4)wheredenotesthevectorofbusvoltageanglesandVdenotesthevectorofbusvoltagemagnitudes.Thebasesolutionfor=0isfoundviaapowerflow.Then,thecontinuationandparameterizationprocessesareapplied.Inpredictionstep,ali

45、nearapproximationisusedbytakinganappropriatelysizedstepinadirectiontangenttothesolutionpath.Therefore,thederivativeofbothsidesofEquation（4）istaken.（5）InordertosolveEquation5,onemoreequationisneededsinceanunknownvariableisaddedtoloadflowequations.Thiscanbesatisfiedbysettingoneofthetangentvectorcompon

46、entsto+1or-1whichisalsocalledcontinuationparameter.Settingoneofthetangentvectorcomponents+1or-1imposesanon-zerovalueonthetangentvectorandmakesJacobiannonsingularatthecriticalpoint.AsaresultEquation5becomes:（6）whereistheappropriaterowvectorwithallelementsequaltozeroexceptthekelementequals1.Atfirstste

47、pischosenasthecontinuationparameter.Astheprocesscontinues,thestatevariablewiththegreatestrateofchangeisselectedascontinuationparameterduetonatureofparameterization.BysolvingEquation6,thetangentvectorcanbefound.Then,thepredictioncanbemadeasfollows:（7）wherethesubscriptp+1”denotesthenextpredictedsoluti

48、on.Thestepsizeischosensothatthepredictedsolutioniswithintheradiusofconvergenceofthecorrector.Ifitisnotsatisfied,asmallerstepsizeischosen.Incorrectionstep,thepredictedsolutioniscorrectedbyusinglocalparameterization.Theoriginalsetofequationisincreasedbyoneequationthatspecifiesthevalueofstatevariablech

49、osenanditresultsin:（8）Whereisthestatevariablechosenascontinuationparameterandisthepredictedvalueofthisstatevariable.Equation（8）canbesolvedbyusingaslightlymodifiedNewton-Raphsonpowerflowmethod.八VoltageStabilityIndicesThediscussedcoefficientmaybetreatedasameasureofvoltagestabilitymarginfromthepointofviewofdemandincrease.AvoltagestabilityindexbasedontheclassicaldQ/dVcriterioncanbeconstructedbyobservingthatastheload