含风电场的电力系统静态电压稳定性分析翻译.docx
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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-StateCharacteristics
thespeed-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-FedInductionMachines
Doubly-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StabilityofaPowerSystem
Powersystemstabilityisunderstoodastheabilitytoregainanequilibriumstateafterbeingsubjectedtoaphysicaldisturbance.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ConnectionsofWindFarms
Althoughthemajorityofwindturbinesaresituatedonland,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InfluenceofWindGeneratorsonPowerSystemStability
Thesynchronousgeneratorisstifflyconnectedtothepowersystemandexhibitsaninherentlyoscillatoryresponsetoadisturbancebecauseitspoweroutputisapproximatelyproportionaltothesineoftherotorangle.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.Recallthatthestabilityofsynchronousgenerator