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

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.Iftherotorspeedisbelowsynchronousspeed，theinductionmachineisoperatingasamotorandiftherotorspeedisabovesynchronousspeed，theinductionmachineisrunningasagenerator.themechanicalpowerandthemechanicaltorquearegivenbytheslip,rotorresistanceandtherotorcurrent.Thespeed-torquecharacteristicoftheinductionmachineisquitelineararoundsynchronousspeed.thetorqueisproportionaltotheinverseoftherotorresistance.Thisimpliesthatitispossibletohaveexternalrotorresistancesconnectedinserieswiththeexistingrotorresistancesofawound-rotorinductionmachine.Bychangingthevalueoftheexternalrotorresistanceitispossibletochangetheslopeofthespeed-torquecharacteristic.Onedisadvantagewiththismethodisthatitisonlypossibletoincreasetheslipusingtheexternalrotorresistances.Thisimpliesthatiftheinductionmachineisrunningasamotor,thenanincreasedrotorresistancewilldecreasetherotorspeed.Ontheotherhand,iftheinductionmachineisrunningasagenerator,theniftherotorresistanceincreases,therotorspeedwillalsoincrease.Beforesemiconductorswereavailable,onewayofadjustingtheslipwastointroduceexternalrotorresistances.Theexternalrotorresistancewillcauseadditionallossesintherotorcircuit.Whensemiconductorsbecameavailableitwaspossibletorecovertheslipotherwisedissipatedintheexternalrotorresistance.Thus,theslippowercanberecoveredintomechanicalorelectricalenergy;therefore,thismethodiscalled"slippowerrecovery."Therotorcurrentmustberectifiedwithadioderectifier.Formotoroperation,therotorcircuitwillseethedioderectifierasaresistanceandthereforethismethodwillworkapproximatelyinthesamewayasfortheexternalrotorresistances.Notethatthedioderectifiercannotbeusedingeneratoroperation.Therectifiedcurrentcouldbeconvertedtomechanicalpowerusingadcmotorcoupledtotheshaftoftheinductionmotororfedbackintothegrid.SinceKramerdriverequireanextradcmotoritisofnointerest,whiletheScherbiusdriveisstillinuse.Themainadvantageofthisconfigurationcomparedtotheexternalrotorresistanceisthatthelossesoftheexternalrotorresistancecanberecovered.Ifbothstatorvoltageandfrequencycanbeadjustedbyaninverter,thetorque-speedcharacteristiccanbeeasilychanged.Whenthespeedisincreasedsothatthestatorvoltagereachesmaximumvoltage,thereisneedforfieldweakening,thestatorvoltageiskeptconstantwhilethefrequencyisstillincreased.二Theinterductionofdoubly-FedInductionMachinesDoubly-fedmachinescanbeusedinvariable-speedconstant-frequencyapplications,suchaswindturbines.Themainadvantageofadoubly-fedmachinecomparedtoasingly-fedforavariable-speedsystemisthereducedratingoftheconverter'spowerrating.Thereductioninpowerratingisdependentonthespeedrangeofthedrive.Thestandarddoubly-fedinductionmachineisawoundrotorinductionmachineequippedwithsliprings.Thestatorcircuitisconnecteddirectlytothegridwhiletherotorcircuitiscontrolledbyaninverterviasliprings.Thecascadeddoubly-fedinductionmachineconsistsoftwodoubly-fedinductionmachineswithwoundrotors,thatareconnectedmechanicallythroughtherotorandelectricallythroughtherotorcircuits.Thestatorcircuitofoneofthemachinesisdirectlyconnectedtothegridwhiletheothermachine'sstatorisconnectedviaaninvertertothegrid.Sincetherotorvoltagesofbothmachinesareequal,itispossibletocontroltheinductionmachinethatisdirectlyconnectedtothegridwiththeotherinductionmachine.Itisdoubtfulwhetheritispracticaltocombinetwoindividualmachinestoformacascadeddoubly-fedinductionmachine,eventhoughitisthebasicconfigurationofdoubly-fedinductionmachinearrangement.Duetoalargeamountofwindings,thelossesareexpectedtobehigherthanforastandarddoubly-fedinductionmachineofacomparablerating.三StabilityofaPowerSystemPowersystemstabilityisunderstoodastheabilitytoregainanequilibriumstateafterbeingsubjectedtoaphysicaldisturbance.threequantitiesareimportantforpowersystemoperation:(i)anglesofnodalvoltages,alsocalledpowerorloadangles;(ii)frequency;and(iii)nodalvoltagemagnitudes.Thesequantitiesareespeciallyimportantfromthepointofviewofdefiningandclassifyingpowersystemstability.Hencepowersystemstabilitycanbedividedinto:(i)rotor(orpower)anglestability;(ii)frequencystability;and(iii)voltagestability.Aspowersystemsarenonlinear,theirstabilitydependsonboththeinitialconditionsandthesizeofadisturbance.Consequently,angleandvoltagestabilitycanbedividedintosmall-disturbanceandlarge-disturbancestability.Powersystemstabilityismainlyconnectedwithelectromechanicalphenomena.However,itisalsoaffectedbyfastelectromagneticphenomenaandslowthermodynamicphenomena.Hence,dependingonthetypeofphenomena,onecanrefertoshort-termstabilityandlong-termstability.四ConnectionsofWindFarmsAlthoughthemajorityofwindturbinesaresituatedonland,thereisagrowingdemandforwindturbinestobeplacedoffshorewithsomelargewindfarmsnowoperational.Thisdoesnotmeanthatoffshoresitesarealwaysbetterthanthoseonshore,assomeonshoresiteshavebetterwindregimesthansitesoffshore.Acommonproblemtoalloffshoreenergyconversionsystemsistheelectricalcableconnectiontotheonshoresubstation.andthisthenraisesdistanceissuesbecauseallACcableshavehighcapacitanceandthelinechargingcurrentforlongcablerunscanbeveryhigh.whileanumberofindependentcablerunsmaybenecessaryinordertotransmittherequiredpowerfromanoffshorewindfarm.BecauseofthelargecablecapacitanceACcablesarecurrentlylimitedtoadistanceundertheseaofabout100-150kmwiththemaximumratingofthree-coresubmarinecablescurrentlybeingabout200MWat145kV,althoughlargerratingsareunderdevelopment.Generallytheoutputsofanumberofturbinesarecollectedtogetheratanoffshoresubstationforonwardtransmissiontoshore.Oncetheoutputofanumberofturbineshasbeencollected,analternativetoACtransmissiontoshoreistouseDCtransmission.NewDCtransmissiontechnologyusesIGBTvoltagesourceconvertersatthesendingend(andpossiblyalsoatthereceivingend)allowingtotalcontrolatthesendingend.Forhigherpowers,conventionalDCtechnologyusingGTOscanbeused.CurrentlyoffshorewindfarmsaresufficientlyclosetoshorethatACcablescanbeused,althoughanumberofcablesmaybenecessarytotransmittherequiredpower.Onepracticalpointtonoteisthatthedistancetoshorealsoincludestheshore-basedcableruntotheshoresubstation.Insomesituationsthiscanbesubstantial.Theproblemsassociatedwithtransferringelectricalpowertoshorefromoffshorewindfarmsisalsofacedbytidalstreamgeneratorsandwavegenerators.Tidalstreamgeneratorstendtoberelativelyclosetoshore,althoughlayingcablesinthestrongcurrentswheretheseturbinesaresituatedisnotstraightforward.Waveenergyisinitsinfancywiththelargeamountsofresourceavailable.Harnessingthisenergyandtransferringittoshoreposesasignificantchallenge.五InfluenceofWindGeneratorsonPowerSystemStabilityThesynchronousgeneratorisstifflyconnectedtothepowersystemandexhibitsaninherentlyoscillatoryresponsetoadisturbancebecauseitspoweroutputisapproximatelyproportionaltothesineoftherotorangle.Forsmallvaluesoftherotorangle,powerisproportionaltotheangleitselfwhichproducesspring-likeoscillations.Ontheotherhand,squirrel-cage(fixed-speed)inductiongeneratorsarecoupledtothegridlessstifflythansynchronousgenerators.thetorqueofafixedspeedinductiongeneratorisproportionaltothespeeddeviation(slip)henceprovidinginherentdampingofoscillations.Thispositiveinfluenceiscounteractedbythevulnerabilityoffixed-speedinductiongeneratorstosystemfaults.DampingduetovariablespeedDFIGsdependsverymuchontheparticularcontrolstrategyemployed.theDFIGshavegoodcontrolcapabilitiesduetothepossibilityofcontrollingboththemagnitudeandphaseoftheinjectedvoltage.Thismakesitpossibletodesignapowersystemstabilizerthatimprovesthedampingofpowerswingswithoutdegradingthequalityofvoltagecontrolprovided.Fullyratedconvertersystemseffectivelydecouplethegeneratorfromthegrid,sotheyofferaverygoodpossibilityofimprovingthedampingofpowerswings.Hencethegeneralconclusionisthatapartialreplacementoftraditionalthermalplantsemployingsynchronousgenerators,whichexhibitarelativepoornaturaldamping,byrenewablegenerators,whichexhibitabetterdamping,willimprovethedampingofelectromechanicalswings.Thiseffectwillbecounterbalancedtosomeextentbythehighlyvariablenatureofrenewablesourcesthemselves,suchaswind,marineorsolar,buttheirvariabilitymaybeeffectivelymanagedbyeitherusingenergystorageorpartloadingoneoftheturbinesinafarmandusingitssparecapacitytosmoothpoweroscillations.Thenetworkeffectofreplacinglargetraditionalgeneratorsbyrenewableoneswilllargelydependonthesysteminquestion.Recallthatthestabilityofsynchronousgeneratorsdeterioratesiftheyarehighlyloaded,remoteandoperatewithalow,orevenleading,powerfactor.Ifrenewableplantsareconnectedclosertotheloads,thenthetransmissionnetworkswillbelessloaded,whichwillreducereactivepowerconsumptionbythesystemandthevoltageswillrise.Thiseffectcanbecompensatedbyreactivepowerdevices,suchasreactorsorstaticVARcompensators,butthiswouldrequireadditionalinvestment.Ifthatisdeemeduneconomicalandtheremainingsynchronousgeneratorsareusedforreactivepowercompensation,theiroperatingpointswouldmovetowardscapacitiveloading(leadingpowerfactor)sotheirdynamicpropertiesmightdeteriorate.Asthenumberofsynchronousgeneratorsremaininginoperationisreducedduetoincreasedpenetrationofrenewables,theiroverallcompensationcapabilitieswillalsobereduced.Hencetheoveralleffectmightbeadeteriorationofthedynamicpropertiesofthesystem.Ontheotherhand,iftherenewablesourcesarelocatedfurtherawayfromthemainloadcentres,thenpowertransfersoverthetransmissionnetworkwillincrease.Highertransferswillmeanlargervoltageangledifferencesbetweennetworknodesanddeterioratedsystemdynamicproperties(smallerstabilitymargins).Increasedpenetrationofrenewablesmightalsoaffectfrequencystability.Duetoitsconstruction,awindplanthassmallerinertiaandspeedsothatkineticenergystoredinitisreducedbyafactorofapproximately1.5whencomparedwithatraditionalplantofthesamerating.Thereductioninstoredkineticenergywillhaveaneffectonsystemoperationandsecuritybecauseoftheamplitudeoffrequencyvariations.六VoltageStabilityVoltagestabilityistheabilityofapowersystemtomaintainsteadyacceptablevoltagesatallbusesinthesystemundernormaloperatingconditionsandafterbeingsubjectedtoadisturbance.Voltagestabilitycanbeattainedbysufficientgenerationandtransmissionenergy.Generationandtransmissionunitshavedefinitecapacitiesthatpeculiartothem.Theselimitsshouldnotbeexceededinahealthypowersystem.Voltagestabilityproblemariseswhenthesystemisheavilyloadedthatcausestogobeyondlimitationsofpowersystem.Apowersystementersastateofvoltageinstabilitywhenadisturbance,increaseinloaddemandpowerorchangeinsystemconditioncausesaprogressiveanduncontrollabledeclineinvoltage.Themainfactorcausinginstabilityistheinabilityofthepowersystemtomeetthedemandforreactivepower.Themainreasonforvoltageinstabilityisthelackofsufficientreactivepowerinasystem.Generatorreactivepowerlimitsandreactivepowerrequirementsintransmissionlinesarethemaincausesofinsufficientreactivepower.Synchronousgeneratorsarethemaindevicesforvoltagecontrolandreactivepowercontrolinpowersystems.Involtagestabilityanalysisactiveandreactivepowercapabilitiesofgeneratorsplayanimportantrole.Theactivepowerlimitsareduetothedesignoftheturbineandtheboiler.Therefore,activepowerlimitsareconstant.Reactivepowerlimitsofgeneratorsaremorecomplicatethanactivepowerlimits.Therearethreedifferentcausesofreactivepowerlimitsthatare;statorcurrent,over-excitationcurrentandunder-excitationlimits.Thegeneratorfieldcurrentislimitedbyover-excitationlimnerinordetoavoiddamageinfieldwinding.Infact,reactivepowerlimitsarevoltagedependent.However,inloadflowprogramstheyaretakentobeconstantinordertosimplifyanalysis.七AnalysisofvoltagestabilityThemostcommonmethodsusedinvoltagestabilityanalysisarecontinuationpowerflow,pointofcollapse,minimumsingularvalueandoptimizationmethods.Inthisstudy,continuationpowerflowmethodiswidelyusedinvoltagestabilityanalysis.Sovoltagestabilitycanbeanalyzedbyusingcontinuationpowerflow.TheJacobianmatrixofpowerflowequationsbecomessingularatthevoltagestabilitylimit.Continuationpowerflowovercomesthisproblem.Continuationpowerflowfindssuccessiveloadflowsolutionsaccordingtoaloadscenario.Itconsistsofpredictionandcorrectionsteps.Fromaknownbasesolution,atangentpredictorisusedsoastoestimatenextsolutionforaspecifiedpatternofloadincrease.ThecorrectorstepthendeterminestheexactsolutionusingNewton-Raphsontechniqueemployedbyaconventionalpowerflow.Afterthatanewpredictionismadeforaspecifiedincreaseinloadbaseduponthenewtangentvector.Thencorrectorstepisapplied.Thisprocessgoesuntilcriticalpointisreached.Thecriticalpointisthepointwherethetangentvectoriszero.Incontinuationloadflow,firstpowerflowequationsarereformulatedbyinsertingaloadparameterintotheseequations.Injectedpowerscanbewrittenforthei-busofann-bussystemasfollows（1）（2）wherethesubscriptsGandDdenotegenerationandloaddemandrespectivelyontherelatedbus.Inordertosimulatealoadchange,aloadparameterisinsertedintodemandpowersand.(3)andareoriginalloaddemandsoni-buswhereasandaregivenquantitiesofpowerschosentoscaleappropriately.AftersubstitutingnewdemandpowersinEquation(2)toEquation(3),newsetofequationscanberepresentedas:(4)wheredenotesthevectorofbusvoltageanglesandVdenotesthevectorofbusvoltagemagnitudes.Thebasesolutionfor=0isfoundviaapowerflow.Then,thecontinuationandparameterizationprocessesareapplied.Inpredictionstep,alinearapproximationisusedbytakinganappropriatelysizedstepinadirectiontangenttothesolutionpath.Therefore,thederivativeofbothsidesofEquation（4）istaken.（5）InordertosolveEquation5,onemoreequationisneededsinceanunknownvariableisaddedtoloadflowequations.Thiscanbesatisfiedbysettingoneofthetangentvectorcomponentsto+1or-1whichisalsocalledcontinuationparameter.Settingoneofthetangentvectorcomponents+1or-1imposesanon-zerovalueonthetangentvectorandmakesJacobiannonsingularatthecriticalpoint.AsaresultEquation5becomes:（6）whereistheappropriaterowvectorwithallelementsequaltozeroexceptthekelementequals1.Atfirststepischosenasthecontinuationparameter.Astheprocesscontinues,thestatevariablewiththegreatestrateofchangeisselectedascontinuationparameterduetonatureofparameterization.BysolvingEquation6,thetangentvectorcanbefound.Then,thepredictioncanbemadeasfollows:（7）wherethesubscript"p+1”denotesthenextpredictedsolution.Thestepsizeischosensothatthepredictedsolutioniswithintheradiusofconvergenceofthecorrector.Ifitisnotsatisfied,asmallerstepsizeischosen.Incorrectionstep,thepredictedsolutioniscorrectedbyusinglocalparameterization.Theoriginalsetofequationisincreasedbyoneequationthatspecifiesthevalueofstatevariablechosenanditresultsin:（8）Whereisthestatevariablechosenascontinuationparameterandisthepredictedvalueofthisstatevariable.Equation（8）canbesolvedbyusingaslightlymodifiedNewton-Raphsonpowerflowmethod.八VoltageStabilityIndicesThediscussedcoefficientmaybetreatedasameasureofvoltagestabilitymarginfromthepointofviewofdemandincrease.AvoltagestabilityindexbasedontheclassicaldQ/dVcriterioncanbeconstructedbyobservingthatastheload