文献翻译——供电可靠性提高

第1页外文文献资料ImprovementofpowersupplyreliabilityCaseStudy:ZambiaTerenceTambatambaMasterofScienceinElectricPowerEngineeringSubmissiondate:June2009NorwegianUniversityofScienceandTechnologyDepartmentofElectricalPowerEngineering1.ProblemDescriptionThereliabilityofthepowersupplysysteminZambiaisnotsatisfactory.Tworecentpowerblackoutshavebeenserious.ThepurposeofthisThesisistoinvestigatethesystembymeansofstabilityanalysis,contingencyanalysisandreviewofprotectionsettingswiththeaimofimprovingthereliabilityofthesystem.TheworkincludesdatacollectionfromZESCOLtd,Zambiatheconsequencesoftheinitialfaultprogressivelyspreadintotherestofthepowersystem.Theroleplayedbypowersystemprotectioninpowerblackoutsituationsiscrucial.1.1.Ablackoutincidentmaybecausedasaresultofthefollowingprotectionlimitations:1)Failuretooperateinthepresenceofanactualfault;2)Failuretooperaterapidlywithintherequireddesignoperatingtimes;3)Failuretoprovideaselectivetripinstructiontospecificsystemcircuitbreakers;Thereisacommoncascadingprocessfordifferentblackouts,whichcanbedescribedasfollows:Systemstatebeforetheblackout:Beforetheblackout,systemparametersusuallyremainwithintheirnormaloperatingreliabilityranges.第2页Atthesametime,somenoticeabledeviationsthatcouldpotentiallyweakenthesystem,suchashighelectricitydemands,heavypowerflows,depressedvoltages,andfrequencyvariations,etc,couldbeobserved.Somescheduledmaintenancesonthenearbygeneratorsand/ortransmissionfacilitiesmayalsohappenbeforetheblackouts.Contingencyconditions:Beforetheblackouts,thesystemmaybeadditionallyweakenedbyunscheduledoutages,suchasseveraltransmissionline,transformerandgeneratoroutagesduetothefaults.Thoseconditionsmovethesystemtoamorestressfulstage.Initiatingevents:Atacertainpointoftheblackoutdevelopment,atriggeringeventhappened.Triggeringeventisthepointseparatingaperiodwheremultiplecontributingbutnotdirectfactorsforfinalblackoutareaccumulated,fromthedirectsequenceofeventswithclearcause/effectrelationships.Transientstateprogressionwithfastsuccession:Inthisstage,thesystembeginstolosemajorpartswhichresultsinbiggerpowerswing,overloadorlowvoltage.Thecomponentsbegintotriponebyoneinaveryshorttime.Uncontrollablesystemseparation,angleinstability,andvoltagecollapsemayoccur.Asaresult,asignificantloadlossmaybeinflicted.Finallarge-scaleblackoutisreachedinaveryshortperiod.Fromtheabovedescription,onecanseethatthecascadingblackoutisaresultofaccumulationofachainofcontingenciesandsystemreactions1.2.Tworecentpowerblackoutshavebeenserious.1.2.1.Blackoutof19thJanuary2008TheblackoutwasinitiatedbyadisturbanceintheZimbabweangridwhichresultedinthelossofamajorloadandshutdownoftheKaribaNorthBankpowerstation.ThiscausedoverloadingandsubsequenttrippingofthegeneratorsatKafueGorgeandVictoriaFallspowerstationsresultingintheblackout.Thesequenceofeventsleadingtotheblackoutcan1.2.2.Blackoutof21stJanuary2008Theblackoutwasinitiatedbyaspurioustrippingontheonly330kV第3页transmissionlineavailableatthetimelineNo.2fromKaribaNorthBankpowerstationtoLeopardsHillsubstation.ThefailureofthislinecompletelyisolatedKaribaNorthBankPowerStationfromthenationalgridbecauseon30thDecember2007,atowerontheonlyotherlineLineNo.1hadcollapsedduetoheavyrainsmakingthislineunavailable.ThissituationcausedoverloadingofKafueGorgeandVictoriaFallspowerstationsandsubsequenttrippingonunderfrequencyprotection.Theblackoutof22ndJanuary2008wascausedbyacollapseofthesystemvoltageduetoinsufficientgenerationcapacity.TheZambiansystemhadbeenisolatedfromZimbabweandKaribaNorthBankPowerStationwasnotavailableonthisdaybecausebothlinesthattransmitpowerfromthisstationwereoutofservice,asindicatedinabove.KafueGorgePowerStationwasoperatingatmaximumavailablegenerationwithoutanyreservemarginatallandasthedemandforpowerincreased2.SMALLSIGNALANDTRANSIENTSTABILITY2.1.StabilityDefinitionInordertoclearlydeterminethegoalsofthisresearch,theconceptof“stability”mustbedefinedsincethistermrepresentsdifferentconceptstodifferentpersonsinvolvedwithpowersystemstability.Adefinitiongivenisasfollows:“Powersystemstabilitymaybebroadlydefinedasthatpropertyofapowersystemthatenablesittoremaininastateofoperatingequilibriumundernormaloperatingconditionsandtoregainanacceptablestateofequilibriumafterbeingsubjectedtoadisturbance.”Fromthisgeneraldefinition,twocategoriesofstabilityarederived:small-signalandtransientstability.Small-signalstabilityistheabilityofthesystemtoreturntoanormaloperatingstatefollowingasmalldisturbance.Investigationsinvolvingthisstabilityconceptusuallyinvolvetheanalysisofthelinearizedstatespaceequationsthatdefinethepowersystemdynamics.Transientstabilityistheabilityofthesystemtoreturntoanormaloperatingstatefollowingaseveredisturbance,suchasasingleormulti-第4页phaseshort-circuitorageneratorloss.Undertheseconditions,thelinearizedpowersystemmodeldoesnotusuallyapplyandthenonlinearequationsmustbeuseddirectlyfortheanalysis.2.2.SmallSignalStabilitySincesmallsignalstabilityinvolvesperturbationsaroundtheoperatingpoint,analysiscanthereforebedonebylinearizingasetofstateequationsthatdescribethepowersystem.2.3.TransientStabilityTherealpowersystemisadynamicsystemandthenormaloperationconditionmaybealteredbycertaindisturbancescausedbyfaults,loadrejection,lineswitching,andlossofexcitation.3.CONTINGENCYANALYSIS3.1.IntroductionPowersystemsareplannedandoperatedsothatthemostprobableandcriticalcontingenciescanbesustainedwithoutaninterruptionoraqualityreduction.Thepowersystemshouldbeabletocontinueitsoperationdespitesuddenoutageofaproductionunit,transmissionline,transformer,compensationdevice,etc.Outagesofpowersystemequipmentaretypicallyduetofaults(shortcircuitsandearthfaults),overloads,malfunctions(falsesettingsoroperationactions)orbreakdownofequipment.Theoccurrenceofdisturbancescannotbepredicted,thusthesecurityofthepowersystemneedstobeguaranteedbeforehand.Thesecurityofthepowersystemiscommonlydefinedbasedon(n-1)criteriai.e.thesystemshouldwithstandanysinglecontingency.Asystemconsistingofncomponentsshouldbeabletooperatewithanycombinationofn-1components,thusforanysinglecomponentoutage.Thiscriterionplaysanimportantroleinpreventingmajordisturbancesfollowingseverecontingencies.Theuseofcriteriaensuresthatthesystemwillatworst,transitfromthenormalstatetothealertstate.The第5页probabilityofn-2contingencyincreasese.g.whentheweatherisbadandthetwotransmissioncircuitsareplacedonthesametowers,orwhenageneratormayberelayedoutduetoalineoutage.Thesecurityanalysisisrequiredtoguaranteethepowersystemssecureoperationinallconditionsandatalloperationpoints.Thepurposeofpowersystemplanningistoensurepowersystemadequacy.Theanalysisisbasedonforecastcases.Onlinesecurityassessmentisneededinthepowersystemoperationtoguaranteesecuritymomentarily,becausenotallpossiblefuturescenarioscanbecheckedbeforehand.Thepurposeofpowersystemoperationistoguaranteethatasingledisturbancecannotcausecascadingoutagesandfinallyatotalblackout.3.2.OperatingStatesofaPowerSystemTheoperatingconditionofapowersystemmaybeclassifiedas:(i)normal,(ii)emergencyand(iii)restorative.Thenormalstateisone,inwhichthetotaldemandonthesystemismetbysatisfyingalltheoperatingconstraints.Contingencies,suchastheoutageofageneratingunit,shortcircuitandsubsequenttrippingofaline,lossofatransformer,etc.canleadtotwotypesofemergencyconditions.Inthefirsttype,thesystemremainsstablebutoperateswiththeviolationofsomeoftheoperatingconstraints.Thus,whiletheconsumersdemandismet,anabnormalvoltageandfrequencyconditionmayarise;loadinglimitsofsomelinesandequipmentmaybeviolated,andsoon.Thistypeofemergencycanbetoleratedforacertainperiod.Inthesecondtype,thepowersystembecomesunstableand39hencetheloadscannotbefullysupplied.Thesecondtypeofemergencythuscausesaviolationofboththeloadingandtheoperatingconstraints,andunlesscorrectiveactionistakenimmediately,thesystemfacestheriskofatotalshutdown.Intherestorativestate,correctiveactionistakensothatthesystemgoesbackeithertonewnormalstateortothepreviousnormalstate.Thisstateischaracterizedbytheinterruptionoftheconsumersdemand,bringingintooperationofrapid-startunits.Generatorsoutagesarenotconsideredherefortwomainreasons1)ThecontingencyanalysismoduleinNETBASdoesnotsupportgenerator第6页outage.2)Theoccurrenceofentirepowerstationoutageisrare.Onlysectionsofitmaybetakenoutforscheduledmaintenance.4.TRANSMISSIONLINEPROTECTION4.1.GeneralTheprotectiverelaysystem,whichisshowninthezoomedareainFig1,isthemostimportantcomponentinthepowersystemtopreservethereliabilityofsystemoperation.Itdetectsthefaultsusingitsfaultdiagnosismechanismandremovesthefaultedcomponentbyitsassociatedcircuitbreakeractioninashorttimetoavoiddamageofthesystemequipment.Theprotectionsystemfortransmissionlinesisveryimportantsincethetransmissionlinesaremostlyextendedacrosslargegeographicareatocarrythepowerfromsourcestoloads.Theycaneasilyexperienceafaultduetothelightningthatcauseslossofinsulation.Theperformanceofprotectionsystemismeasuredbyseveralcriteriaincludingreliability,selectivity,speedofoperation,etc.Reliabilityhastwoaspects:dependabilityandsecurity.Dependabilityisdefinedas“thedegreeofcertaintythatarelaysystemwilloperatecorrectlywhenthereisafaultonthesystem”.Security,“relatestothedegreeofcertaintythatarelayorrelaysystemwillnotoperateincorrectlywhenthereisnofaultonthesystem”.Foraweakenedsystemthatalreadylostseveralcomponents,lossofreliabilityduetotherelaymal-operationwillhavealargeimpactonthesystemthatmaycontributetothecascadingblackout.Therearetwokindsofrelayunintendedoperations:Relayfailstooperate.Thissituationisrelativerare.Butitisveryharmfultothesystemstabilitywhenithappens.Eventhoughthefaultisclearedbythebackuprelaysinadelayedtime,therearehealthycomponentsremovedfromthesystem.Thiscanresultinmorepowerflowsurges,overloads,andvoltageproblemsforaweakenedsystem.Relayoperatesinanon-faultsituation.Thissituationismorecommon第7页inmostofblackoutsinvolvingthedistancerelaysunintendedoperations.Forexample,therelaysmayobservealowvoltageandahighcurrent,atthetimeduringtheoverload,powerswingorlowvoltage.Tripofthehealthycomponentswillalsoresultinmorepowerflowsurges,overloads,andvoltageproblemsforaweakenedsystem.Theeventmayunfoldandspreadout.Eachtransmissionlineprotectiverelay(distancerelay)hasitsowndesignatedareaknownasprimaryzone,andusuallyitstillhastheopportunitytooperateinoverreachedzonestoprovidebackupprotectiontoanadjacenttransmissionlinesection.Toensuretheselectivityoftransmissionlineprotectionsystems,therelaysneedtobecoordinatedwiththebackuprelaystooperateonlywhentheprimaryrelayfailstoclearthefault.Hence,selectivityisimportanttoassuremaximumservicecontinuityandminimumsystemdisconnection.The45speedofoperationindicateshowfasttherelaycanisolateafaultedarea.Usuallyfortransmissionlineprotection,thehigh-speedrelayisonethatoperatesinlessthan20milliseconds.Notinallsituationstheveryhigh-speedoperationispreferred.Therelaymusthavetheabilitytodifferentiatefaultandothertolerabletransientsverywellbeforeissuinghigh-speedoperation.Themostcommonlyusedschemefortransmissionlineprotectionisthedistancerelay.ThebasicprincipleisshowninFig26.Thevoltageandcurrentmeasuredthroughvoltagetransformer(VTorPT)andcurrenttransformer(CT)aretheinputsforprotectiverelays.ThedistancerelayalgorithmistryingtoextractthefundamentalFig26Basicprinciplesofdistancerelayfrequencyphasorofvoltageandcurrentsignals.Thenthroughthecalculationofcertainnonlinearratioofvoltageandcurrentphasors,theapparentimpedanceseenbytherelayisobtainedandcomparedtothepresetthresholds.Iftheimpedancefallsintotheprotectedzones,therelaywillassumeafaultoccurredandwillsendatripsignaltothecircuitbreakeronthetransmissionlinetodisconnectthefaultedline.Oncetriggered,theimpedancecalculationiscontinuouslyiteratedusingthemovingdatawindow.Therelayalgorithmisfixedbydesign.Whentherelayisinstalledinthesystem,themostimportanttaskisto第8页determinethethresholds(settings).Thesettingsareobtainedbycomprehensiveshort-circuitsystemstudiesinapredefinedsystemoperatingcondition.Toensuretheprotectionsystemmaintainsdependabilitywhenprotectingtheequipmentasfastaspossible,abackupschemeisprovidedforeachrelay.46Thedistancerelayprincipleisstraightforwardandusuallyperformsreliably.Thatisthereasonwhythisprincipleisstilldominantlyusedintheindustry,althoughtherelayhardwarehasadvancedthroughthetechnologiesofelectromechanical,solidstate,andmicroprocessor.4.2.TypesofTransmissionLineRelaysIngeneral,thetransmissionlinefaultsareassociatedwithincreasedcurrentsanddecreasedvoltages.OtherchangesoftheACquantitiesinoneofthefollowingparametersmayalsooccur:phaseanglesofcurrentandvoltagephasors,harmoniccomponents,activeandreactivepower,frequencyofthepowersystem,etc18.Thoseparameterscanbetheinputsoftherelaystodetectthefaults.Theoperatingprinciplesoftherelaysinuseontransmissionlinesmaybeclassifiedasfollows:MagnitudeRelays:Theserelaysarebasedonthecomparisonofthemagnitudeofoneormoreoperatingquantitiestothethreshold.Forexample,theover-currentrelayrespondstothechangesinthemagnitudeoftheinputcurrent.Theload-sheddingrelayrespondstothechangesofthesystemfrequency.DirectionalRelays:TheserelaysarebasedonthecomparisonofthephaseanglebetweentwoACinputs.Thecomparisoncanbebasedoncurrentphasorandvoltagephasor,andalsooncurrentphasorandanothercurrentphasor.RatioRelays:Theserelaysarebasedonthecomparisonoftheratiooftwophasorstothethresholds.Theratiooftwophasorsarecomplexnumber,thereforethethresholdshouldbesetinacomplexplane.Atypicalexampleofaratiorelayisthedistancerelay.DifferentialRelays:Theserelaysarebasedonthealgebraicsumoftwoormoreinputs.Inageneralform,thoseinputsmaybethecurrentsentering第9页(orleaving)aspecificprotectionzone.AccordingtotheKirchhoffslaw,thealgebraicsumshouldbeclosetozerowhenthereisnointernalfaultandshouldbeabigvaluewhenthereisaninternalfault.5.CONCLUSIONS5.1.TransientStabilityStudyTransientstabilityofthegeneratorsintheZescosystemwasstudied.ThesystemwasmodeledinSIMPOWbywritingascriptcalledanoptpowfileforloadflowsimulationsandthedynpowfilefortransientstabilitysimulations(seeappendixD).Inordertostudythedynamicbehaviorofthegeneratorswhensubjectedtoalargedisturbance,thegeneratorsweremodeledastype2A(i.e.modelwithonefieldwinding,onedamperwindingind-axisandonedamperwindinginq-axis.Saturationexcluded).GeneratorparametersobtainedfromtheZescocataloguewereused.ThesimulationsfocusedonthedynamicbehaviorofZescogeneratorswhensubjectedtoathreephaseshortcircuitappliedatthreeselectedbusesnamelyKaribaNorth,LeopardsHillandKafuegorge.Thesebuseswereselectedbecausetheyareconsideredcriticaltothesystem.Thedurationoftheshortcircuitineachcasewasvarieduntilstabilityinatleastonesetofgeneratorswaslost.Resultsshowthatwiththefaultdurationof100milliseconds,allgeneratorsinthesystemwereabletoregainsynchronismafterthefaultwasclearedbutwiththedurationextendedto200millisecondsthegeneratorsatVictoriaFallspowerstation,whichisthesmallestofthethree,werethefirsttolosesynchronism.Thesimulationswerecarriedoutatpeakloadcondition.Itcanthereforebeconcludedthatatthecurrentloadinglevel,thecriticalclearingtimeshouldbelessthan200milliseconds.Thisguaranteesthatnogeneratorwilllosesynchronism.5.2.ContingencyStudyThisstudywasimplementedusingsoftwarecalledNETBAS.Resultsshowthatalthoughcertainconstraintsareviolatedwhenacontingencyisapplied(becausetheanalysisisdoneatpeakloadcondition),theZescosystemcan第10页operateunder(N-1)criteria.5.3.ProtectionSystemThestudyproposedthatwhencalculatingthedistancerelayprotectionsettings,theeffectofzerosequencemutualcouplingthatexistsbetweenparallelcircuitsshouldbeconsidered.Asseeninsection6.5.1theperformanceofconventionaldistancerelaysonparallellinesisnegativelyimpactedbythezerosequencemutualcouplingeffectbetweenlines.Usingsettingsbasedupontheworstcasescenarioresultsinsub-optimalperformanceoftherelaysunderotheroperatingconditions.Ithasbeenshownthatitisfeasibletoachieveanoptimaldistancerelayperformanceonparallellinesbyaccessingmultipleadditionallocallyavailable63signalsandadaptrelayoperationbasedonthesignalsavailabilityandthelinestatus.Theadaptiveprotectionschemecouldprovideanenhanceddistanceprotectionforparallellinesasnoremotesignalsarerequiredforthescheme.Theadaptationtothesignalavailabilityprovidesabuilt-infallbackscheme,whichensuresthereliableoperationoftherelayunderallconditions.Theapplicationofthenewadaptiveschemewouldenhancetheperformanceofdistanceprotectiononparallellines.5.4.OtherConsiderations5.4.1.AutomaticUnderFrequencyLoadSheddingTheabsenceofaneffectiveautomaticunder-frequencyload-sheddingschemecontributedsignificantlytothefailuretocontainthedisturbances.ImplementationofsuchaschemewouldenableZescorespondtothelossofgenerationbyautomaticallyswitchingoffappropriateloads,therebybalancingdemandwiththeavailablegeneration.Zescoshouldconsiderimplementingsuchaschemetoimprovereliabilityofpowersupply.5.4.2.SystemCapacityandSpinningReserveTheothermostimportantfactorforthebuildingupoffaultsintototalblackoutshastodowiththefailuretomaintainagenerationspinningreserve(sparecapacityatgeneration).第11页WhileitsunderstoodthatZescoiscurrentlyunderpressuretominimizeload-shedding,theabsenceofaspinningreservecontributestotheinabilityofthesystemtocontainthelargedisturbances.Zescoshouldreviewsystemoperationandensurethatareasonablereservecapacityisalwaysmaintained.Untilsignificantnewgenerationcapacityisdeveloped,therewillbedifficultiesandcompromisesinrunningtheZambiansystem.HoweverplansarecurrentlyunderwaytobuildanewpowerstationatKafuelower.Thiswillgreatlyimprovepowersupplyreliabilityandreduceloadshedding.5.4.3.SystemMonitoringItiscurrentlydifficulttoreconstructsomeoftheeventsthatoccurduringthedisturbancesbecausetherecordersandprotectiverelaysonthesystemarenottime-synchronized.Itistherefore,recommendedthatZescotakesimmediatestepstoacquireequipmentthatisneededtosynchroniestheso-calledtimestampingonalleventrecorders.第12页中文翻译稿供电可靠性提高案例研究：赞比亚特伦斯Tambatamba科学的电力工程硕士提交日期：2009年6月科学技术的挪威大学电力工程系1.问题描述在赞比亚的供电系统的可靠性不令人满意。最近两次停电很严重的。本论文的目的是通过稳定性分析，应变分析和审查来提高系统的可靠性，对保护设置的方式进行调查研究。工作包括从泽思科有限公司尚比亚数据收集初始故障的后果，逐步扩散到电力系统的其余部分。在停电的情况下电力系统保护所起的作用是至关重要的。1.1.停电事件可能引起的下列保护限制的结果：1）未在实际故障的存在下进行操作;2）未在规定的设计工作时间内迅速地进行操作;3）未能提供选择性跳闸指令到特定的系统断路器;有一个共同的级联过程对于不同的停电，它可以被描述如下：系统状态前的停电：停电之前，系统参数通常保持其正常运行可靠性的范围之内。与