专业英语13章 电气工程英语翻译.docx
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专业英语13章 电气工程英语翻译.docx
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专业英语13章电气工程英语翻译
12.3GroundingofElectricalSystems
Ingeneral,mostelectricalsystemsmustbegrounded.Thepurpose
Fig.12.4Secondaryhigh-voltageradialdistributionsystem
ofgroundingistolimitthemagnitudeofvoltagecausedbylighting,momentarysurges,andaccidentalcontactwithhighervoltages.Systemgroundsmustbeseaweedtoprovideapathofminimumimpedanceinordertoensuretheoperationofover-currentdeviceswhenagroundfaultoccurs.Currentshouldnotflowthroughthegroundingconductorduringnormaloperation.
Direct-currentsystemsgenerallyhavethegroundingconductorconnectedtothesystematthesupplystation,andnotattheindividualservice.Alternating-currentsystems,ontheetherhand,mustbegroundedondiesupplysideofthemaindisconnectaleachindividualservice.Forspecificinformationanthelocationandmethyloffunding,refertoNECArticle250.
(a)Secondaryhigh-voltagedistributionsystem;high-voltageradical.low-voltageloop
(b)Consumerdistributionsystemwithhigh-voltageandlow-voltageloopsFig.12.5
12.4GroundingofElectricalEquipment
Metalconduitandcaseswhichencloseelectricalconductorsmustbegrounded.Iftheungrounded(hot)conductorcomesincontactwithametalenclosurewhichisnotgrounded,avoltagewillbepresentbetweentheenclosureandtheground.Thispresentsapotentialhazard.Personacomicincontactwiththeenclosureandgroundwillcompleteacircuit.
Allnon-current-carryingmetalpartsofelectricalinstallationsshouldbetightlybondedtogetherandconnectedtoagroundingelectrode.Goodelectricalcontinuityshouldbeensuredthroughallmetalenclosures.Thecurrentcausedbyaccidentalgroundswillbeconductedthroughtheenclosures,thegroundingconductor,andthegroundingelectrodetotheearth.Ifthecurrentisfalseenough,itmillcausetheover-currentdevicetoopen.
12.5GroundFaultProtection
Aground-faultprotector(GFP)isadevicewhichsensesgroundfaultsandopensthecircuitwhenthecurranttogroundreachesapredeterminedvalue.Aground-faultcircuitinterrupter(GFCI)isadevicewhichopensthecircuitwhenverysmallcurrentsflowtoground.
Thereisnowaytodetermineinadvancetheimpedanceofanaccidentalground.Mostcircuitsareprotectedby15A(ampere)orlargerover-currentdevices.Iftheimpedanceofagroundfaultislowenough,suchdeviceswillopenthecircuit.Whataboutcurrentsoflessthan15A?
Ithasbeenproventhatcurrentsassmallas50mAthroughtheheart,lungs,orbraincanbefatal.
Electricalequipmentexposedtomoistureorvibrationmaydevelophigh-impedancegrounds.Arcingbetweenaconductorandtheframeofequipmentmaycauseafire,yetthecurrentmaybelessthan1ampere.
Leakagecurrentcausedbydirtand/ormoisturemaytakeplacebetweentheconductorandtheframe.Portabletoolsarefrequentlynotproperlygrounded.andtheonlypathtogroundisthroughthebodyoftheoperator.
Theground-faultcircuitinterrupterwasdevelopedtoprovideprotectionagainstground-faultcurrentsoflessthan15A.theGFCIisdesignedtooperateontwo-wirecircuitsinwhichoneofthetwowiresisgrounded.Thestandardcircuitvoltagesare120Vand277V.Thetimeittakesoperatedependsuponthevalueoftheground-faultcurrent.Smallcurrentsof20mAorlessmayflowforupto5sbeforethecircuitisopened.Acurrentof20mAwillcausetheGECItooperateinlessthan0.04s.Thistime/currentelementprovidesasufficientmarginofsafetywithoutnuisancetripping.
TheGFCIoperatesontheprinciplethatanequalamountofcurrentisflowingthoughthetwowires.Whenagroundfaultoccurs,sameofthecurrantflowingthroughtheungrounded(hot)wiredoesnotflowthroughthegroundedwire;itcompletesthecircuitthoughtheaccidentalground.TheGFCIsensesthedifferenceinthevalueofcurrentbetweenthetwowiresandopensthecircuit.GFCIsmaybeincorporatedintocircuitbreakersinstalledintheline,orincorporatedintoareceptacleoutletorequipment.
Ground-faultprotectorsaregenerallydesignedforusewithcommercialand/orindustrialinstallations.Theyprovideprotectionagainstground-faultcurrentsfrom2A(specialtypesgoaslowas50mA)upto2000A.GFPsaregenerallyinstalledonthemain,submain,and/orfeederconductors.GFClsareinstalledinthebranchcircuits.GFPsaregenerallyusedforthree-wire,single-phaseandforthree-phaseinstallations,whileGEClsareusedfortwo-wire,single-phasecircuits.
Aground-faultprotectorinstalledonsupplyconductorsmustencloseallthecircuitconductors,includingtheneutral,ifpresent.Whenoperatingundernormalconditions,allthecurrenttoendfromtheloadflowsthroughthecircuitconductors.Thealgebraicsumofthefluxproducedbythesecurrentsiszero.Whenaphase-to-groundfaultoccurs,thefaultcurrentsreturnsthroughthegroundingconductor.Underthisconditionanalternatingfluxisproducedwithinthesensingdevice.Whenthefluxcurrentreachesapredeterminedvalue,themagneticfluxcausesarelaytoactuateacircuitbreaker.
SometimestheGFPisinstalledonthegroundingconductorofthesystem.Underthiscondition,theunitsensestheamountofphase-to-groundcurrentflowinginthegroundingconductor.Whenthecurrentexceeds
thesettingoftheGFP,itwillcausethecircuitbreakertoopen.
Theground-faultprotectorisactuallyaspeciallydesignedcurrenttransformerconnectedtoasolid-staterelay.
12.Three-PhaseSystems
Thevariousthree-phasesystemsinnormalusewillliedescribed.Underidealconditions,thesesystemsoperateinperfectbalance,andifaneutralconductorispresentitcarrieszerocurrent.Inactualpractice,perfectlybalancedsystemsareseldomencountered.Theelectricalworker,therefore,mustbetocalculatevaluesofcurrentandvoltageinunbalancedsystems.Single-phaseloadsarefrequentlysuppliedfromthree-phasesystem.Thesingle-phaseloadrequirementsvaryconsiderably,makingitvirtuallyimpossibletomaintainaperfectbalance.
Inabalancedthree-phasesystem,thecurrentsinthethreelinesareequal.Thecurrentsinthethreephasesarealsoequal.Inotherwords,ILX=ILY=ILZandIp=Ip=Ip.if,however,ILX≠ILF≠ILZ,thenIPX≠IPY≠IPZandthesystemisunbalanced(seeFig.12.6).
Tocalculatethelinecurrentsinanunbalancedthree-phasesystem,themethodinthefollowingexamplemaybeused.
Example1
Threepureresistance,single-phaseloadsareconnectedinadeltaconfigurationacrossathree-phasesupply,asillustratedinFig.12.6.LoadXrequires30A,loadYrequires50A,andloadZrequires80A.Calculatethecurrentthrougheachlinewire.
Example1appliestoloadsof100percentpowerfactorconnectedindelta.Withloadsofdifferentpowerfactors,thephaseanglewillvaryfrom120°.Forawyeconnection,thelinecurrentisequaltothephasecurrent.
Someconnectionsmaybeacombinationofsinge-phaseandthree-phaseloads.Undertheseconditions,thephaseanglebetweenthree-phaseloadandthesingle-phaseloadmustbeconsidered.
12.7HarmonicEffectofFluorescentLightingFixtures
MostdistributionsystemsintileUnitedStatesandCanadaoperateonafrequencyof60Hz.certaintypesofelectricalequipmentproducesecondaryfrequenciesaremultiplesofthesupplyfrequency.Thesesecondaryfrequenciesarecalledharmonics.Forexample,thesecondharmonicof60Hzis120Hz,thethirdharmonicis180Hz,andsoon
Thealienatingfluxdevelopedbytransformers,usedintheballastsoffluorescentlightingfixtures,producesavoltagewhichhasafrequencyof180hertz.Thisresultsinanadditionalcurrentflowinginthesupplyconductors.Thevalueofthecurrentinthephaseconductorsisusualabout25percentofthesupplycurrent.Thisthirdharmoniccurrentaddstothesupplycurrent,causingagreaterheatingeffectintheconductors.Thisincreasedheatingeffectisrathersmall,possiblyinthevicinityof3-5percent.
Theeffectontheneutralconductorisquitedifferent.Theharmoniccurrentsfromthephaseconductorsaddtogether,causingalargeincreaseintheneutralcurrent.Theheatingeffectis75%-80%greaterthanifthethirdharmoniccurrentdidnotexist.
CAUTION:
Wheninstallingsupply,feeder,andbranchcircuitconductorsforheavyfluorescentloads,thesizeoftheneutralconductorshouldbeatleastequaltothatofthephaseconductors.
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