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电力系统中的高电压技术High-VoltageTechnologyInPowerSystemsTopics•Insulation•Grounding•Overvoltage•LightningShieldingInsulation1•Insulationisrequiredtokeepelectricalconductorsseparatedfromeachotherandfromothernearbyobjects.•Thechoiceofmaterialisdictatedbytherequirementsoftheparticularapplicationandbycost.Insulation2•Thecoilsofgeneratorsandmotorsmaybeinsulatedwithvarioustapes.•Theinsulationonpowertransformerwindingsiscommonlypapertapeandpressboardoperatedunderoil.Insulation3•Overheadlinesaresupportedonporcelaininsulators.Betweenthesupportsairservesasinsulation.Porcelainischosenbecauseofitsresistancetodeteriorationwhenexposedtotheweather,itshighdielectricstrength,anditsabilitytowashcleanbyrain.Insulation4•Thecostofequipmentandthesystemasawholeiscloselyrelatedtotheinsulationlevelwhichhasbeenadoptedforthenetwork.Thechoiceofinsulationlevelshouldtakeintoaccountboththeprobabilityofoccurrenceandtheseverityofstressthatthesystemmaybesubjectedto.500KV开关场switchyard500KV变压器transformer220KV开关场switchyard220KV开关场switchyard电力电缆powercables电力电缆powercable钢芯铝绞线aluminumconductorsteelreinforced架空输电线overheadtransmissionline架空输电线overheadtransmissionlineInsulationCoordination•Electricsystems’insulationdesignershavetwooptionsavailabletothem:chooseinsulationlevelsforcomponentsthatwouldwithstandovervoltages,deviseprotectivedevicesthatcouldlimitovervoltagesthere.Grounding•Groundingisdefinedasaconductingconnection,whetherintentionaloraccidental,bywhichanelectriccircuitorequipmentisconnectedtotheearth,ortosomeconductingbodyofrelativelylargeextentthatservesinplaceoftheearth.Grounding•Themostimportantreasonforgroundingissafety.Twoimportantaspectstogroundingrequirementswithrespecttosafetyandonewithrespecttopowerqualityarefollows:—Personnelsafety—Toassureprotectivedeviceoperation—Noisecontrol接地网施工constructionofgroundgrid接地网施工constructionofgroundgridOvervoltages•Theovervoltagescanbedividedintoexternalovervoltagesandinternalovervoltagesaccordingtothelocationsofsources.•Externalovervoltagesaregeneratedbysourcesthatexternaltothepowersystemnetwork.Theirmagnitudeisessentiallyindependentofthesystem.ExternalOvervoltages•Lightningovervoltagesarecharacterizedbyveryhighpeakcurrentsandrelativelylowenergycontent.Theyareresponsiblefornearlyhalfofallshortcircuitsonlinesinsystemsof300kvandabove.Upto300kvthesysteminsulationhastobedesignedprimarilytowithstandlightningsurges.InternalOvervoltages•Internalovervoltagesareprincipallydeterminedbythesystemconfigurationanditsparameters.Theyaremainlyduetoswitchingoperationsandfollowingfaultconditions.•Accordingtotheirduration,whichrangesfromhundredsofmicrosecondstoseveralseconds,threemaincategoriesofinternalovervoltagescanbedefined:InternalOvervoltages•1TransientSwitchingSurges.•2TemporaryOvervoltages.•3.Steady-stateOvervoltages.LightningStrokes•Duringstormyweather,byaprocessnotyetfullyunderstood,achargeseparationtakesplaceinsideclouds,sothatpositivechargesmovetotheupperpartofthecloudwhilenegativechargesstaybelow.Thistransferofelectricchargesetsupanelectricfieldwithinthecloud.LightningStrokes•Thenegativechargeatthebaseofthecloudrepelsthefreeelectronsonthegroundbelow.Consequently,thesurfaceofthegroundbecomespositivelycharged,byinduction.LightningStrokes•Asmoreandmorepositivechargesaremoveupwardwithinthecloud,theelectricfieldbelowthecloudbecomesmoreandmoreintense.Ultimately,itreachesthecriticalionizationlevelwhereairbeginstobreakdown.Ionizationtakesplacesfirstatthetipsofhighbuildings,suchaschurchspires,andthetopofhightrees.LightingStrokeLightingStrokeLightingStrokeLightingStrokesLightingConductorsLightingConductorsLightningShieldingofTransmissionLines•Whenlightningmakesadirecthitonatransmissionline,itdepositsalargeelectriccharge,producinganenormousovervoltagebetweenthelineandtheground.•Thedielectricstrengthofairisimmediatelyexceededandaflashoveroccurs.LightningShieldingofTransmissionLines•Thearcbetweenthelineandthegroundproducesahighlyionizedpathwhichbehaveslikeaaconductingshort-circuit.Consequently,thenormalAClinevoltageimmediatelydeliversalargeACcurrentthatfollowstheionizedpath.Thisfollow-throughcurrentmaysustainthearcuntilthecircuitbreakersopenattheendofline.LightningShieldingofTransmissionLines•Directhitsonatransmissionlinearerare.Moreoften,lightningwillstriketheoverheadgroundwirethatshieldstheline.Inthelattercase,alocalchargeshillaccumulatesontheline,producingaveryhighlocalovervoltage.OverheadGroundWiresOverheadGroundWiresLightningArrestersTravellingWaves•Theconcentratedchargeimmediatelydividesintotwowavesthatswiftlymoveinoppositedirectionsatclosetothespeedoflight.•Theheightoftheimpulsewaverepresentsthemagnitudeofthesurgevoltagethatexistsfrompointtopointbetweenthelineandtheground.Thepeakvoltagemayattainoneortwomillionvolts.