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青岛大学毕业论文(设计)科技文献翻译院系：自动化工程学院电气工程系专业：电气工程及其自动化班级：2010级电气1班姓名：尹睿指导教师：袁博强2014年5月15日1FaultsonPowerSystemsEachyearnewdesignofpowerequipmentbringaboutincreasedreliabilityofoperation.Nevertheless,equipmentfailuresandinterferencebyoutsidesourcesoccasionallyresultinfaultsonelectricpowersystems.Ontheoccurrenceofafault,currentandvoltageconditionsbecomeabnormal,thedeliveryofpowerfromthegeneratingstationtotheloadsmaybeunsatisfactoryoveraconsiderablearea,andifthefaultedequipmentisnotpromptlydisconnectedfromtheremainderofthesystem,damagemayresulttootherpiecesofoperatingequipment.Afaultyistheunintentionalorintentionalconnectingtogetheroftwoormoreconductorswhichordinarilyoperatewithadifferenceofpotentialbetweenthem.Theconnectionbetweentheconductorsmaybebyphysicalmetalliccontactoritmaybethroughanarc.Atthefault,thevoltagebetweenthetwopartsisreducedtozerointhecaseofmetal-to-metalcontacts,ortoaverylowvalueincasetheconnectionisthroughanarc.Currentsofabnormallyhighmagnitudeflowthroughthenetworktothepointoffault.Theseshort-circuitcurrentswillusuallybemuchgreaterthanthedesignedthermalabilityoftheconditioninthelinesormachinesfeedingthefault.Theresultantriseintemperaturemaycausedamagebytheannealingofconductorsandbythecharringofinsulation.Intheperiodduringwhichthefaultispermittedtoexist,thevoltageonthesysteminthenearvicinityofthefaultwillbesolowthatutilizationequipmentwillbeinoperative.Itisapparentthatthelateconditionsthatexistduringafault,andprovideequipmentproperlyadjustedtoopentheswitchesnecessarytodisconnectthefaultedequipmentfromtheremandingofthesystem.Ordinarilyitisdesirablethatnootherswitchesonthesystemareopened,assuchbehaviorwouldresultinunnecessarymodificationthesystemcircuits.Adistinctionmustbemadebetweenandanoverload.Anoverloadimpliesonlythatloadsgreaterthanthedesignedvalueshavebeenimposedonsystem.Undersuchacircumstancethevoltageattheoverloadpointmaybelow,butnotzero.Thisundervoltageconditionmayextendforsomedistancebeyondtheoverloadpointintotheremainderofthesystem.Thecurrentintheoverloadequipmentarehighandmayexceedthethermaldesignlimits.Nevertheless,suchcurrentsaresubstantiallylowerthaninthecaseofafault.Servicefrequentlymaybemaintained,butatbelow-standardvoltage.Overloadsarerathercommonoccurrencesinhomes.Forexample,ahousewifemightplugfivewaffleironsintothekitchencircuitduringaneighborhoodpart.Suchanoverload,ifpermittedtocontinue,wouldcauseheatingofthewiresfromthepowercenterandmighteventuallystartafire.Topreventsuchtrouble,residentialcircuitsareprotectedbyfusesorcircuitbreakerswhichopenquicklywhencurrentsabovespecifiedvaluespersist.Distributiontransformersaresometimesoverloadsascustomersinstallmoreandmoreappliances.The2continuousmonitoringofdistributioncircuitsisnecessarytobecertainthattransformerssizesareincreasedasloadgrows.Faultsofmanytypesandcausesmayappearonelectricpowersystems.Manyofusinourhomeshaveseenfrayedlampcordswhichpermittedthetwoconductorsofthecordtocomeincontactwitheachother.Whenthisoccurs,thereisaresultingflash,andifbreakerorfuseequipmentfunctionsproperly,thecircuitisopened.Overheadlines,forthemostpart,areconstructedofbareconductors.Therearesometimesaccidentallybroughttogetherbyactionofwind,sleets,trees,cranes,airplanes,ordamagetosupportingstructures.Overvoltagesduetolightingorswitchingnaycauseflashoverofsupportingorfromconductortoconductor.Contaminationoninsulatorssometimesresultsinflashoverevenduringnormalvoltageconditions.Theconductorsofundergroundcablesareseparatedfromeachandfromgroundbysolidinsulation,whichnaybeoil-impregnatedpaperoraplasticsuchpolyethylene.Thesematerialsundergosomedeteriorationwithage,particularlyifoverloadsonthecableshaveresultedintheiroperationatelevatedtemperature.Anysmallvoidpresentinthebodyoftheinsulatingmaterialwillresultsinionizationofthegascontainedtherein,theproductsofwhichreactunfavorablywiththeinsulation.Deteriorationoftheinsulationmayresultinfailureofthematerialtoretainitsinsulatingproperties,andshortcircuitswilldevelopbetweenthecableconductors.Thepossibilityofcablefailureisincreasediflighteningorswitchingproducestransientvoltageofabnormallyhighvaluesbetweentheconductors.Transformerfailuresmaybetheresultofinsulationdeteriorationcombinedwithovervoltageduetolightningorswitchingtransients.Shortcircuitduetoinsulationfailurebetweenadjacentturnsofthesamewindingmayresultfromsuddenlyappliedovervoltage.Majorinsulationmayfail,permittingarcstobeestablishedbetweenprimaryandsecondarywindingsorbetweenwindingandgroundedmetalpartssuchasthecoreortank.Generatorsmayfailduetobreakdownoftheinsulationbetweenadjacentturnsinthesameslot,resultinginashortcircuitinasingleturnofthegenerator.Insulationbreakdownmayalsooccurbetweenoneofthewindingandthegroundedsteelstructureinwhichthecoilsareembedded.Breakdownbetweendifferentwindingslyinginthesameslotresultsinshort-circuitingextensivesectionofmachine.Balancedthree-phasefaults,likebalancedthree-phaseloads,maybehandledonalineto-neutralbasisoronanequivalentsingle-phasebasis.Problemsmaybesolvedeitherintermsofvolts,amperes,andohms.Thehandingoffaultsonsingle-phas