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英文文献CirtcuitBreakersAcircuitbreakerismechanicalswitchingdevicecapableofmaking,andbreakingcurrentsundernormalcircuitconditionsandalsomaking.carringforaspecifiedtime,andmediujminwhichcircuitinterruptionisperformedmaybedesignatedbyasuitableprefix,forexample,air-blastcircuitbreaker,oilcircuitbreaker.Thecircuitbreakerscurrentlyinusecanbedlassifiedintothefollowingcategoriesaccordingtothearc-quenchingprinciples:airswetchesoelciryitbreakers,minmum-oilcircuitbreakers,air-blastcircuitbreakers,themageneticaircircuitbreakers,minimum-oilcircuitbreakers,aer-blastcircuitbreakers,thebyvoltage,insulationlevelcurrent,interruptingcapabilities,transientrecoverycoltage,interruptingtiome,andtripdelay.Thenameplateonacircuitbreakerusuallyindicates:１.Themaximumsteady-statecurrentitcancarry,2.Themaximuminterruptingcurrent,3.Themaximumlinevoltage,4.Theinterruptingtimeincycles,Theinterruptingtimeinmaylastform3to8cyclesona60Hzsystem.Tointerrubtlargecurrentsquickly,wehavetoensurerapidcooling.High-speedinterruptionlunitsthedamagetransmissionlinesandequipmentand,equallyimportant,ithelpstomainmainthestabilityofthesystemwheneveracontingencyoccurs.Themainpartsofacircuitbreakerareusually:arc-quenchingchamber(orinterrupterwithmovingandfixedcontacts)operatingmechanismandsupportingstructures.AirSwitches-Withincreasingcurrentsandvoltages,spring-actiondrivingmechanismsweredevelopedtoreducecontactburingbyfaster-openingoperation.Later,maincontactswerefittedwitharcingcontactsofspecialmaterialandshape,whichopendafterandclosedbeforethemaincontacts.Furtherimprovementsoftheairswitchwerethebursh-typecontactwithawipingandcleaningfunction,theinsulatingbarrierleadingtoarcchutes,andblowoutcoilswithexcellentarc-extinguishingproperties.Thesefeatures,aswellasthehorngapcontact,arestillinuseinlowvoltageasanddebreakers.OilCircuitBreakerAround1900,inordertocopewiththenewrequirementfor“interruptingcapacity”,ACswitcheswereimmersedinatankofoil.Isveryeffectiveinquenchingthearcandestablishingtheopenbreakaftercurrentzero.Deiongrids,oil-blastfeatures,pressure-tightjointsandvents,newoperatingmechanisims,andmultipleinterrupterwereintroucedoverseveraldecadestomaketheoilcircuitbreakerareliableapparatusforsystemvoltageupto362Kv.TypesandConstructionofTransformerAtransformerisadevicethatalternatingcurrentelectricenergyatonevoltagelevelintoalternatingcurrentelectricenergyatanothervoltagelevelthroughtheactionofamagneticfield.Itconsistsoftwoormorecoilswirewrappedaroundacommonferromagneticcore.Thesecoilsare(usually)notdirectlyconnected.Theonlyconnectionbetweenthecoilsisthecommonmagnecticfluxpresenwithinthecore.Oneofthetransformerwindingsisconnectedtoasourceofacelectricpower,andthesecond(andperhapsthird)transformerwindingsupplieselectricpowertoloads.thetransformerwindingconnectedtothepowersouceiscalledtheprimarywindingorinputwinding.andthewindingconnectedtotheloadsiscalledthesecondarywindingorinputwinding.Ifthereisathirdwindingonthetransformer,itiscalledthetertiarywinding.Powertransformerareconstructedononeoftwotypesofcores.onetypeofconstructionconsistsofasimplerectangularlaminatedpieceofsteelwiththetransformerwindingswrappedaroundtwosidesoftherectangle.Thistypeofconstructionisknowascoreform.Theothertypeconsistsofthree-leggedlaminatedcorewiththewindingswrappedaroundthecenterleg.Thistypeofconstructionisknowasshellform.Ineithercase,thecoreisconstructedofthinlaminationselectricallyisolatedformeachotherinorderinordertoreduceeddycurrentstoaminimum.Theprimaryandsecondarywindingsinaphysicaltransformerarewrappedoneontopoftheotherwiththelow-voltagewindinginnermost.Suchanarrangementseverstwopurposes:1.Itsimplifiestheproblemofinsulatingthehigh-voltagewindingfromthecore.2.Itresultsinmuchlessleakagefluxthanwouldbethetwowindingswereseparatedbyadistanceonthecore.Powertransformeraregivenavarietyofdifferentnames,dependingontheiruseinpowersystems.Atransformerconnectedtotheoutputofageneratorandusedtostepitsvoltageuptotransformerlevelsissometimescalledunittransformer.Thetransformeraitheotherendofthetransformerline,whichstepsthevoltagedownfromtransmissionlevelstodistributionlevels,iscalledasubstationtransformer.Finally,thetransformerthattakesthedistributionvoltageandstepsisdowntothefinalvoltageaiwhichthepowerisactuallyusediscalledadistributiontransformer.Allthesedevicesareessentiallythesame-theonlydifferenceamongthemistheirintendeduse.Inadditiontothevariouspowertransformer,twospecial-purposetransformersareusedwithelectricmachineryandpowersystems.Thefirstofthesespecialtransformersisadevicespeciallydesignedtosampleahighvoltageandproducealowsecondaryvoltagedirectlyproportionaltoit.Suchatransformeriscalledapotentialtransformer.Apowertransformeralsoproducesasecondaryvoltagedirectlyproportionaltoitsprimaryvoltage;thedifferencebetweenapotentialtransformerandapowertransformeristhatthepotentialtransformerisdesignedtohandleonlyaverysmallcurrent.Thesecondtypeofspecialtransformerisadevicedesignedtoprovideasecondarycurrent.muchsmallerthanbutdirectlyproportionaltoitsprimarycurrent.Thisdeviceiscalledacurrenttransformer.