电气工程及其自动化专业英语

电气工程及其自动化专业英语SectionIbasicelectriccircuitChapter1IntroductiontoelectriccircuitsNewWordsandExpressions1.electricalcircuitn.电路2.voltagen.电压，伏特3.currentn.电流，通用的,流通的,现在的4.currentflown.电流5.resistorn.电阻，电阻器6.batteryn.电池7.loadn.负载，负荷8.performancen.性能9.circuitdiagramn.电路图10.idealizedmodeln.理想模型Introduction*Asimplecircuitanditscomponents.*Idealizedmodelofthecircuit*Modelcanbechangedifnecessary.*summarizeInelementaryphysicsclassesyouundoubtedlyhavebeenintroducedtothefundamentalconceptsofelectricityandhowrealcomponentscanbeputtogethertoformanelectricalcircuit.Averysimplecircuit,forexample,mightconsistofabattery,somewire,aswitch,andanincandescentlightbulbasshowninFig.1-1.Thebatterysuppliestheenergyrequiredtoforceelectronsaroundtheloop,heatingthefilamentofthebulbandcausingthebulbtoradiatealotofheatandsomelight.Energyistransferredfromasource,thebattery,toaload,thebulb.————Youprobablyalreadyknowthatthevoltageofthebatteryandtheelectricalresistanceofthebulbhavesomethingtodowiththeamountofcurrentthatwillflowinthecircuit.Fromyourownpracticalexperienceyoualsoknowthatnocurrentwillflowuntiltheswitchisclosed.Thatis,foracircuittodoanything,theloophastobecompletedsothatelectronscanflowfromthebatterytothebulbandthenbackagaintothebattery.Andfinally,youprobablyrealizethatitdoesn’tmuchmatterwhetherthereisonefootortwofeetofwireconnectingthebatterytothebulb,butthatitprobablywouldmatterifthereisamileofwirebetweenitandthebulb.AlsoshowninFig.1-1isamodelmadeupofidealizedcomponents.Thebatteryismodeledasanidealsourcethatputsoutaconstantvoltage,VB,nomatterwhatamountofcurrent,i,isdrawn.Thewiresareconsideredtobeperfectconductorsthatoffernoresistancetocurrentflow.Theswitchisassumedtobeopenorclosed.Thereisnoarcingofcurrentacrossthegapwhentheswitchisopened,noristhereanybouncetotheswitchasitmakescontactonclosure.Thelightbulbismodeledasasimpleresistor,R,thatneverchangesitsvalue,nomatterhowhotitbecomesorhowmuchcurrentisflowingthroughit.Fig.1-1(a)Asimplecircuit(b)AnidealizedrepresentationofthecircuitFormostpurposes,theidealizedmodelshowninFig.1-1bisanadequaterepresentationofthecircuit;thatis,ourpredictionofthecurrentthatwillflowthroughthebulbwhenevertheswitchisclosedwillbesufficientlyaccuratethatwecanconsidertheproblemsolved.Theremaybetimes,however,whenthemodelisinadequate.Thebatteryvoltage,forexample,maydropasmoreandmorecurrentisdrawn,orasthebatteryages.————Thelightbulb’sresistancemaychangeasitheatsup,andthefilamentmayhaveabitofinductanceandcapacitanceassociatedwithitaswellasresistancesothatwhentheswitchisclosed,thecurrentmaynotjumpinstantaneouslyfromzerotosomefinal,steadystatevalue.Thewiresmaybeundersized,andsomeofthepowerdeliveredbythebatterymaybelostinthewiresbeforeitreachestheload.Thesesubtleeffectsmayormaynotbeimportant,dependingonwhatwearetryingtofindoutandhowaccuratelywemustbeabletopredicttheperformanceofthecircuit.Ifwedecidetheyareimportant,wecanalwayschangethemodelasnecessaryandthenproceedwiththeanalysis.Thepointhereissimple.Thecombinationsofresistors,capacitors,inductors,voltagesources,currentsources,andsoforth,thatyouseeinacircuitdiagramaremerelymodelsofrealcomponentsthatcomprisearealcircuit,andacertainamountofjudgmentisrequiredtodecidehowcomplicatedthemodelmustbebeforesufficientlyaccurateresultscanbeobtained.Forourpurposes,wewillbeusingverysimplemodelsingeneral,leavingmanyofthecomplicationstomoreadvancedtextbooks.Chapter2DefinitionsofkeyelectricalquantitiesNewWordsandExpressionschargen.vt.电荷；充电nucleusn.原子核（pl.）；nuclearadj.negativen.否定,负数,底片adj.否定的,消极的,负的,阴性的positiveadj.[数]正的adj.[电]阳的ingeneral通常,大体上,一般而言，总的说来algebraicadj.代数的,关于代数学的solutiontothecircuitproblemn.关于电路问题的解法theunitsofpowern.功率的单位directcurrent(dc)n直流电alternatingcurrent(ac)n.交流电sinusoidallyadv.正弦地transistorn.晶体管Part1ChargeandCurrentAnatomconsistsofapositivelychargednucleussurroundedbyaswarmofnegativelychargedelectrons.Thechargeassociatedwithoneelectronhasbeenfoundtobe1.602×10−19coulombs;or,statedtheotherwayaround,onecoulombcanbedefinedasthechargeon6.242×1018electrons.Whilemostoftheelectronsassociatedwithanatomaretightlyboundtothenucleus,goodconductors,likecopper,havefreeelectronsthataresufficientlydistantfromtheirnucleithattheirattractiontoanyparticularnucleusiseasilyovercome.Theseconductionelectronsarefreetowanderfromatomtoatom,andtheirmovementconstitutesanelectriccurrent.Inawire,whenonecoulomb’sworthofchargepassesagivenspotinonesecond,thecurrentisdefinedtobeoneampere(abbreviatedA),namedafterthenineteenth-centuryphysicistAndr’eMarieAmp`ere.Thatis,currentiisthenetrateofflowofchargeqpastapoint,orthroughanarea:i=dq/dt(1.1)Ingeneral,chargescanbenegativeorpositive.Forexample,inaneonlight,positiveionsmoveinonedirectionandnegativeelectronsmoveintheother.Eachcontributestocurrent,andthetotalcurrentistheirsum.Byconvention,thedirectionofcurrentflowistakentobethedirectionthatpositivechargeswouldmove,whetherornotpositivechargeshappentobeinthepicture.Thus,inawire,electronsmovingtotherightconstituteacurrentthatflowstotheleft,asshowninFig.1-2.Fig.1-2Byconvention,negativechargesmovinginonedirectionconstituteapositivecurrentflowintheoppositedirectionWhenchargeflowsatasteadyrateinonedirectiononly,thecurrentissaidtobedirectcurrent,ordc.Abattery,forexample,suppliesdirectcurrent.Whenchargeflowsbackandforthsinusoidally,itissaidtobealternatingcurrent,orac.IntheUnitedStatestheacelectricitydeliveredbytesofacanddcareshowninFig.1-3.(a)(b)Fig.1-3(a)Steady-statedirectcurrent(dc)(b)Alternatingcurrent(ac)Part2Kirchhoff’sCurrentLawTwoofthemostfundamentalpropertiesofcircuitswereestablishedexperimentallyacenturyandahalfagobyaGermanprofessor,GustavRobertKirchhoff(1824–1887).Thefirstproperty,knownasKirchhoff’scurrentlaw(abbreviatedKCL),statesthatateveryinstantoftimethesumofthecurrentsflowingintoanynodeofacircuitmustequalthesumofthecurrentsleavingthenode,whereanodeisanyspotwheretwoormorewiresarejoined.Thisisaverysimple,butpowerfulconcept.Itisintuitivelyobviousonceyouassertthatcurrentistheflowofcharge,andthatchargeisconservative—neitherbeingcreatednordestroyedasitentersanode.Unlesschargesomehowbuildsupatanode,whichitdoesnot,thentherateatwhichchargeentersanodemustequaltherateatwhichchargeleavesthenode.ThereareseveralalternativewaystostateKirchhoff’scurrentlaw.ThemostcommonlyusedstatementsaysthatthesumofthecurrentsflowingintoanodeiszeroasshowninFig.1-4a,inwhichcasesomeofthosecurrentsmusthavenegativevalueswhilesomehavepositivevalues.EquallyvalidwouldbethestatementthatthesumofthecurrentsleavinganodemustbezeroasshowninFig.1-4b(againsomeofthesecurrentsneedtohavepositivevaluesandsomenegative).Finally,wecouldsaythatthesumofthecurrentsenteringanodeequalsthesumofthecurrentsleavinganode(Fig.1-4c).Theseareallequivalentaslongasweunderstandwhatismeantaboutthedirectionofcurrentflowwhenweindicateitwithanarrowonacircuitdiagram.Currentthatactuallyflowsinthedirectionshownbythearrowisgivenapositivesign.Currentsthatactuallyflowintheoppositedirectionhavenegativevalues.Fig.1-4IllustratingvariouswaysthatKirchhoff’scurrentlawcanbestated(a)Thesumofthecurrentsintoanodeequalszero(b)Thesumofthecurrentsleavingthenodeiszero(c)ThesumofthecurrentsenteringanodeequalsthesumofthecurrentsleavingthenodeNotethatyoucandrawcurrentarrowsinanydirectionthatyouwant—thatmuchisarbitrary—butoncehavingdrawnthearrows,youmustthenwriteKirchhoff’scurrentlawinamannerthatisconsistentwithyourarrows,ashasbeendoneinFig.1-4.Thealgebraicsolutiontothecircuitproblemwillautomaticallydeterminewhetherornotyourarbitrarilydetermineddirectionsforcurrentswerecorrect.Example1.1UsingKirchhoff’sCurrentLawAnodeofacircuitisshownwithcurrentdirectionarrowschosenarbitrarily.Havingpickedthosedirections,i1=−5A,i2=3A,andi3=−1A.WriteanexpressionforKirchhoff’scurrentlawandsolvefori4.Solution.ByKirchhoff’scurrentlaw,i1+i2=i3+i4−5+3=−1+i4sothati4=−1AThatis,i4isactually1Aflowingintothenode.Notethati2,i3,andi4areallenteringthenode,andi1istheonlycurrentthatisleavingthenode.Part3Kirchhoff’sVoltageLawElectronswon’tflowthroughacircuitunlesstheyaregivensomeenergytohelpsendthemontheirway.That“push”ismeasuredinvolts,wherevoltageisdefinedtobetheamountofenergy(w,joules)giventoaunitofcharge,v=dw/dq(1-2)A12-Vbatterythereforegives12joulesofenergytoeachcoulombofchargethatitstores.Notethatthechargedoesnotactuallyhavetomoveforvoltagetohavemeaning.Voltagedescribesthepotentialforchargetodowork.Whilecurrentsaremeasuredthroughacircuitcomponent,voltagesaremeasuredacrosscomponents.Thus,forexample,itiscorrecttosaythatcurrentthroughabatteryis10A,whilethevoltageacrossthatbatteryis12V.Otherwaystodescribethevoltageacrossacomponentincludewhetherthevoltagerisesacrossthecomponentordrops.Thus,forexample,forthesimplecircuitinFig.1-1,thereisavoltageriseacrossthebatteryandvoltagedropacrossthelightbulb.Voltagesarealwaysmeasuredwithrespecttosomething.Thatis,thevoltageofthepositiveterminalofthebatteryis“somanyvolts”withrespecttothenegativeterminal;or,thevoltageatapointinacircuitissomeamountwithrespecttosomeotherpoint.InFig.1-5,currentthrougharesistorresultsinavoltagedropfrompointAtopointBofVABvolts.VAandVBarethevoltagesateachendoftheresistor,measuredwithrespecttosomeotherpoint.Fig.1-5ThevoltagedropfrompointAtopointBisVAB,whereVAB=VA−VBThereferencepointforvoltagesinacircuitisusuallydesignatedwithagroundsymbol.Whilemanycircuitsareactuallygrounded—thatis,thereisapathforcurrenttoflowdirectlyintotheearth—somearenot(suchasthebattery,wires,switch,andbulbinaflashlight).Whenagroundsymbolisshownonacircuitdiagram,youshouldconsiderittobemerelyareferencepointatwhichthevoltageisdefinedtobezero.Fig.1-6pointsouthowchangingthenodelabeledasgroundchangesthevoltagesateachnodeinthecircuit,butdoesnotchangethevoltagedropacrosseachcomponent.ThesecondofKirchhoff’sfundamentallawsstatesthatthesumofthevoltagesaroundanyloopofacircuitatanyinstantiszero.ThisisknownasKirchhoff’svoltagelaw(KVL).JustaswasthecaseforKirchhoff’scurrentlaw,therearealternative,butequivalent,waysofstatingKVL.Wecan,forexample,saythatthesumofthevoltagerisesinanyloopequalsthesumofthevoltagedropsaroundtheloop.ThusinFig.1-6,thereisavoltageriseof12Vacrossthebatteryandavoltagedropof3VacrossR1andadropof9VacrossR2.————Noticethatitdoesn’tmatterwhichnodewaslabeledgroundforthistobetrue.JustaswasthecasewithKirchhoff’scurrentlaw,wemustbecarefulaboutlabelingandinterpretingthesignsofvoltagesinacircuitdiagraminordertowritetheproperversionofKVL.Aplus(+)signonacircuitcomponentindicatesareferencedirectionundertheassumptionthatthepotentialatthatendofthecomponentishigherthanthevoltageattheotherend.Again,aslongasweareconsistentinwritingKirchhoff’svoltagelaw,thealgebraicsolutionforthecircuitwillautomaticallytakecareofsigns.Part5SummaryofPrincipalElectricalQuantitiesThekeyelectricalquantitiesalreadyintroducedandtherelevantrelationshipsbetweenthesequantitiesaresummarizedinTable1-1.Sinceelectricalquantitiesvaryoversuchalargerangeofmagnitudes,youwilloftenfindyourselfworkingwithverysmallquantitiesorverylargequantities.Forexample,thevoltagecreatedbyyourTVantennamaybemeasuredinmillionthsofavolt(microvolts,μV),whilethepowergeneratedbyalargepowerstationmaybemeasuredinbillionsofwatts,orgigawatts(GW).Todescribequantitiesthatmaytakeonsuchextremevalues,itisusefultohaveasystemofprefixesthataccompanytheunits.ThemostcommonlyusedprefixesinelectricalengineeringaregiveninTable1-2.Table1-1KeyElectricalQuantitiesandRelationshipsElectricalQuantitySymbolUnitAbbreviationRelationshipChargeqcoulombCq=∫idtCurrentiampereAi=dq/dtVoltagevvoltVv=dw/dqPowerpjoule/secondJ/sp=dw/dtorwattWEnergywJouleJw=∫pdtorwatt-hourWhTable1-2CommonPrefixesSmallQuantitiesLargeQuantitiesQuantityPrefixSymbolQuantityPrefixSymbol10−310−610−910−12MilliMicroNanopicoMµNp1031061091012KiloMegaGigateraKMGTPart6IdealVoltageSourceandIdealCurrentSourceElectriccircuitsaremadeupofarelativelysmallnumberofdifferentkindsofcircuitelements,orcomponents,whichcanbeinterconnectedinanextraordinarilylargenumberofways.Atthispointinourdiscussion,wewillconcentrateonidealizedcharacteristicsofthesecircuitelements,realizingthatrealcomponentsresemble,butdonotexactlyduplicate,thecharacteristicsthatwedescribehere.Anidealvoltagesourceisonethatprovidesagiven,knownvoltagevs,nomatterwhatsortofloaditisconnectedto.Thatis,regardlessofthecurrentdrawnfromtheidealvoltagesource,itwillalwaysprovidethesamevoltage.Notethatanidealvoltagesourcedoesnothavetodeliveraconstantvoltage;forexample,itmayproduceasinusoidallyvaryingvoltage—thekeyisthatvoltageisnotafunctionoftheamountofcurrentdrawn.AsymbolforanidealvoltagesourceisshowninFig.1-7.Aspecialcaseofanidealvoltagesourceisanidealbatterythatprovidesaconstantdcoutput,asshowninFig.1-8.Arealbatteryapproximatestheidealsource;butascurrentincreases,theoutputdropssomewhat.Toaccountforthatdrop,quiteoftenthemodelusedforarealbatteryisanidealvoltagesourceinserieswiththeinternalresistanceofthebattery.Anidealcurrentsourceproducesagivenamountofcurrentisnomatterwhatloaditsees.AsshowninFig.1-9,acommonlyusedsymbolforsuchadeviceiscirclewithanarrowindicatingthedirectionofcurrentflow.Whileabatteryisagoodapproximationtoanidealvoltagesource,thereisnothingquitesofamiliarthatapproximatesanidealcurrentsource.Sometransistorcircuitscomeclosetothisidealandareoftenmodeledwithidealizedcurrentsources.SectionIITheelectricpowersystemChapter1BriefIntroductiontoTheElectricPowerSystemPart1MinimumPowersystemNewWordsandExpressionsMinimuma最小primemovern原动机generatorn发电机loadn负载furnacen炉膛boilern锅炉fissionablen可裂变的fissionablematerial核燃料reactorn反应堆nuclearreactor核反应堆elevationn高度，海拔internalcombustionengine内燃机steam-driventurbine汽轮机hydraulicturbine水轮机convertv变换，转换shaftn传动轴，轴torquen力矩servomechanismn伺服机构*Elementsofaminimumelectricpowersystem*Typesofenergysource*Typesofprimemover*Typesofelectricalload*FunctionsofthecontrolsystemAminimumelectricpowersystemisshowninFig.1-1,thesystemconsistsofanenergysource,aprimemover,agenerator,andaload.Theenergysourcemaybecoal,gas,oroilburnedinafurnacetoheatwaterandgeneratesteaminaboiler;itmaybefissionablematerialwhich,inanuclearreactor,willheatwatertoproducesteam;itmaybewaterinapondatanelevationabovethegeneratingstation;oritmaybeoilorgasburnedinaninternalcombustionengine.Theprimemovermaybeasteam-driventurbine,ahydraulicturbineorwaterwheel,oraninternalcombustionengine.Eachoneoftheseprimemovershastheabilitytoconvertenergyintheformofheat,fallingwater,orfuelintorotationofashaft,whichinturnwilldrivethegenerator.Theelectricalloadonthegeneratormaybelights,motors,heaters,orotherdevices,aloneorincombination.Probablytheloadwillvaryfromminutetominuteasdifferentdemandsoccur.Thecontrolsystemfunctions（are）tokeepthespeedofthemachinessubstantiallyconstantandthevoltagewithinprescribedlimits,eventhoughtheloadmaychange.Tomeettheseloadconditions,itisnecessaryforfuelinputtochange,fortheprimemoverinputtovary,andforthetorqueontheshaftfromtheprimemovertochangeinorderthatthegeneratormaybekeptatconstantspeed.Inaddition,thefieldcurrenttothegeneratormustbeadjustedtomaintainconstantoutputvoltage.Thecontrolsystemmayincludeamanstationedinthepowerplantwhowatchesasetofmetersonthegeneratoroutputterminalsandmakesthenecessaryadjustmentsmanually.Inamodernstation,thecontrolsystemisaservomechanismthatsensesgenerator-outputconditionsandautomaticallymakesthenecessarychangesinenergyinputandfieldcurrenttoholdtheelectricaloutputwithincertainspecifications.Part2MoreComplicatedSystems*Foreword*Casesofpowersystemwithoutcircuitbreaker*PowersystemwithcircuitbreakerNewWordsandExpressions1.associateda联接的2.circuitn电路3.circuitbreakern断路器4.deenergizevt切断，断电5.deenergizedadj不带电的6.outagen停电7.diagramn简图8.switchoutof退出来，断开9.switchoffv切断，关闭Inmostsituationstheloadisnotdirectlyconnectedtothegeneratorterminals.Morecommonlytheloadissomedistancefromthegenerator,requiringapowerlineconnectingthem.Itisdesirabletokeeptheelectricpowersupplyattheloadwithinspecifications.However,thecontrolsarenearthegenerator,whichmaybeinanotherbuilding,perhapsseveralmilesaway.Ifthedistancefromthegeneratortotheloadisconsiderable,itmaybedesirabletoinstalltransformersatthegeneratorandattheloadend,andtotransmitthepoweroverahigh-voltageline(Fig.1-2).Forthesamepower,thehigher-voltagelinecarrieslesscurrent,haslowerlossesforthesamewiresize,andprovidesmorestablevoltage.Insomecasesanoverheadlinemaybeunacceptable.Insteaditmaybeadvantageoustouseanundergroundcable.Withthepowersystemstalkedabove,thepowersupplytotheloadmustbeinterruptedif,foranyreason,anycomponentofthesystemmustbemovedfromserviceformaintenanceorrepair.Additionalsystemloadmayrequiremorepowerthanthegeneratorcansupply.Anothergeneratorwithitsassociatedtransformersandhigh-voltagelinemightbeadded.Itcanbeshownthattherearesomeadvantagesinmakingtiesbetweenthegenerators(1)andattheendofthehigh-voltagelines(2and3),asshowninFig.1-3.Thissystemwilloperatesatisfactorilyaslongasnotroubledevelopsornoequipmentneedstobetakenoutofservice.Theabovesystemmaybevastlyimprovedbytheintroductionofcircuitbreakers,whichmaybeopenedandclosedasneeded.Circuitbreakersaddedtothesystem,Fig.1-4,permitselectedpieceofequipmenttoswitchoutofservicewithoutdisturbingtheremainderofsystem.Withthisarrangementanyelementofthesystemmaybedeenergizedformaintenanceorrepairbyoperationofcircuitbreakers.Ofcourse,ifanypieceofequipmentistakenoutofservice,thenthetotalloadmustbecarriedbytheremainingequipment.Attentionmustbegiventoavoidoverloadsduringsuchcircumstances.Ifpossible,outagesofequipmentarescheduledattimeswhenloadrequirementsarebelownormal.Fig.1-5showsasysteminwhichthreegeneratorsandthreeloadsaretiedtogetherbythreetransmissionlines.Nocircuitbreakersareshowninthisdiagram,althoughmanywouldberequiredinsuchasystem.Chapter2FaultsonPowerSystemNewWordsandExpressions1.faultn故障2.interferencen干扰，防碍3.exceedvt超出，超过4.abnormaln异常的，不规则的5.intentionaln故意的6.feed(fed)给。。。馈电7.utilizationequipment用电设备8.inoperative无法使用的9.modification变更，调整10.substantiallyadj事实上，实际上 11.annealingn退火，煅烧，热处理12.magnituden幅值Part1FaultsanditsDamageEachyearnewdesignsofpowerequipmentbringaboutincreasedreliabilityofoperation.Nevertheless,equipmentfailuresandinterferencebyoutsidesourcesoccasionallyresultinfaultsonelectricpowersystems.Ontheoccurrenceofafault,currentandvoltageconditionsbecomeabnormal,thedeliveryofpowerfromthegeneratingstationstotheloadsmaybeunsatisfactoryoveraconsiderablearea,andifthefaultedequipmentisnotpromptlydisconnectedfromtheremainderofthesystem,damagemayresulttootherpiecesofoperatingequipment.Afaultistheunintentionalorintentionalconnectingtogetheroftwoormoreconductors,whichordinarilyoperatewithadifferenceofpotentialbetweenthem.Theconnectionbetweentheconductorsmaybebyphysicalmetalliccontactoritmaybethroughanarc.Atthefault,thevoltagebetweenthetwopartsisreducedtozerointhecaseofmetal-to-metalcontacts,ortoverylowvalueincasetheconnectionisthroughanarc.Currentsofabnormallyhighmagnitudeflowthroughthenetworktothepointoffault.Theseshort-circuitcurrentswillusuallybemuchgreaterthanthedesignedthermalabilityoftheconductorsinthelinesormachinesfeedingthefault.Theresultantriseintemperaturemaycausedamagebytheannealingofconductorsandbythecharringofinsulation.Intheperiodduringwhichthefaultispermittedtoexist,thevoltageonthesysteminthenearvicinityofthefaultwillbesolowthatutilizationequipmentwillbeinoperative.Itisapparentthatthepowersystemdesignermustanticipatepointsatwhichfaultsmayoccur,beabletocalculateconditionsthatexistduringafault,andprovideequipmentproperlyadjustedtoopentheswitchesnecessarytodisconnectthefaultedequipmentfromtheremainderofthesystem.Ordinarilyitisdesirablethatnootherswitchesonthesystemareopened,assuchbehaviorwouldresultinunnecessarymodificationofthesystemcircuits.Part2OverloadAdistinctionmustbemadebetweenafaultandanoverload.Anoverloadimpliesonlythatloadsgreaterthandesignedvalueshavebeenimposedonsystem.Undersuchacircumstancethevoltageattheoverloadpointmaybelow,butnotzero.Thisundervoltageconditionmayextendforsomedistancebeyondtheoverloadintotheremainderofthesystem.Thecurrentsintheoverloadequipmentarehighandmayexceedthethermaldesignlimits.Nevertheless,suchcurrentsaresubstantiallylowerthaninthecaseofafault.Servicefrequentlymaybemaintained,butatbelow-standardvoltage.Overloadsarerathercommonoccurrencesinhomes.Forexample,ahousewifemightplugfivewaffleironsintothekitchencircuitduringaneighborhoodparty.Suchanoverload,ifpermittedtocontinue,wouldcauseheatingofthewiresfromthepowercenterandmighteventuallystartafire.Topreventsuchtrouble,residentialcircuitsareprotectedbyfusesorcircuitbreakers,whichopenquicklywhencurrentsabovespecifiedvaluespersist.Distributiontransformersaresometimesoverloadedascustomersinstallmoreandmoreappliances.Thecontinuousmonitoringofdistributioncircuitsisnecessarytobecertainthattransformersareincreasedasloadgrows.NewWordsandExpressions1.flashovern闪络2.lightningn雷电，闪电3.oil-impregnatedpapervt油浸纸4.polyethylenen聚乙稀5.deteriorationn老化6.voidn空洞，空隙7.ionizationn电离，游离8.turnn一匝线圈9.coren铁芯10.slotn槽11.embedvt嵌入12.coiln线圈13.windingn绕组14.extinguishvt熄灭15.reclosingn重合闸Part3VariousFaultsFaultsofmanytypesandcausesmayappearonelectricpowersystem.Manyofusinourhomeshaveseenfrayedlampcordswhichpermittedthetwoconductorsofthecordtocomeincontactwitheachother.Whenthisoccurs,thereisaresultingflash,andifbreakerorfuseequipmentfunctionsproperly,thecircuitisopened.1Overheadlines,forthemostpart,areconstructedofbareconductors.Thesearesometimesaccidentallybroughttogetherbyactionofwind,sleet,trees,cranes,airplanes,ordamagestosupportingstructures.Overvoltagesduetolightningorswitchingmaycauseflashoverofsupportingorfromconductortoconductor.Contaminationoninsulatorssometimesresultinflashoverevenduringnormalvoltageconditions.2Theconductorsofundergroundcablesareseparatedfromeachotherandfromgroundbysolidinsulation,whichmaybeoil-impregnatedpaperoraplasticsuchaspolyethylene.Thesematerialsundergosomedeteriorationwithage,particularlyifoverloadsonthecableshaveresultedintheiroperationatelevatedtemperature.Anysmallvoidpresentinthebodyoftheinsulatingmaterialwill