电气自动化专业外文文献及翻译短路电流电气类.docx
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电气自动化专业外文文献及翻译短路电流电气类.docx
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电气自动化专业外文文献及翻译短路电流电气类
外文参考文献及翻译
姓名:
院系:
电气工程及自动化学院
专业:
电气工程及其自动化
班级:
07电41
Short-circuitcurrent
1TermsandDefinitions
ThefollowingtermsanddefinitionscorrespondlargelytothosedefinedinIEC60909.Refertothisstandardforalltermsnotusedinthisbook.
Thetermsshortcircuitandgroundfaultdescribefaultsintheisolationofoperationalequipmentwhichoccurwhenlivepartsareshuntedoutasaresult.
●Causes:
1.Overtemperaturesduetoexcessivelyhighovercurrents.
2.Disruptivedischargesduetoovervoltages.
3.Arcingduetomoisturetogetherwithimpureair,especiallyoninsulators.
●Effects:
1.Interruptionofpowersupply.
2.Destructionofsystemcomponents.
3.Developmentofunacceptablemechanicalandthermalstressesinelectricaloperationalequipment.
●Shortcircuit:
AccordingtoIEC60909,ashortcircuitistheaccidentalorintentionalconductiveconnectionthrougharelativelylowresistanceorimpedancebetweentwoormorepointsofacircuitwhicharenormallyatdifferentpotentials.
●Shortcircuitcurrent:
AccordingtoIEC60909,ashortcircuitcurrentresultsfromashortcircuitinanelectricalnetwork.
Itisnecessarytodifferentiateherebetweentheshortcircuitcurrentatthepositionoftheshortcircuitandthetransferredshortcircuitcurrentsinthenetworkbranches.
●Initialsymmetricalshortcircuitcurrent:
Thisistheeffectivevalueofthesymmetricalshortcircuitcurrentatthemomentatwhichtheshortcircuitarises,whentheshortcircuitimpedancehasitsvaluefromthetimezero.
●Initialsymmetricalshortcircuitapparentpower:
Theshortcircuitpowerrepresentsafictitiousparameter.Duringtheplanningofnetworks,theshortcircuitpowerisasuitablecharacteristicnumber.
●Peakshortcircuitcurrent:
Thelargestpossiblemomentaryvalueoftheshortcircuitoccurring.
●Steadystateshortcircuitcurrent:
Effectivevalueoftheinitialsymmetricalshortcircuitcurrentremainingafterthedecayofalltransientphenomena.
●DCaperiodiccomponent:
Averagevalueoftheupperandlowerenvelopecurveoftheshortcircuitcurrent,whichslowlydecaystozero.
●Symmetricalbreakingcurrent:
Effectivevalueoftheshortcircuitcurrentwhichflowsthroughthecontactswitchatthetimeofthefirstcontactseparation.
●Equivalentvoltagesource:
Thevoltageatthepositionoftheshortcircuit,whichistransferredtothepositive-sequencesystemastheonlyeffectivevoltageandisusedforthecalculationoftheshortcircuitcurrents.
●Superpositionmethod:
Thesuperpositionmethodconsidersthepreviousloadofthenetworkbeforetheoccurrenceoftheshortcircuit.Itisnecessarytoknowtheloadflowandthesettingofthetransformerstepswitch.
●Voltagefactor:
RatiobetweentheequivalentvoltagesourceandthenetworkvoltageUn,dividedby3.
●Equivalentelectricalcircuit:
Modelforthedescriptionofthenetworkbyanequivalentcircuit.
●Far-from-generatorshortcircuit:
ThevalueofthesymmetricalACperiodiccomponentremainsessentially
constant.
●Near-to-generatorshortcircuit:
ThevalueofthesymmetricalACperiodiccomponentdoesnotremainconstant.Thesynchronousmachinefirstdeliversaninitialsymmetricalshortcircuitcurrentwhichislargerthantwicetheratedcurrentofthesynchronousmachine.
●Positive-sequenceshortcircuitimpedance:
Theimpedanceofthepositive-sequencesystemasseenfromthepositionoftheshortcircuit.
●Negative-sequenceshortcircuitimpedance:
Theimpedanceofthenegative-sequencesystemasseenfromthepositionoftheshortcircuit.
●Zero-sequenceshortcircuitimpedance
Theimpedanceofthezero-sequencesystemasseenfromthepositionoftheshortcircuit.Threetimesthevalueoftheneutralpointtogroundimpedanceoccurshere.
●Shortcircuitimpedance:
Impedancerequiredforcalculationoftheshortcircuitcurrentsatthepositionoftheshortcircui
p?
?
?
t.
Shortcircuitpathinthepositive-sequencesystem
Forthesameexternalconductorvoltages,athree-poleshortcircuitallowsthreecurrentsofthesamemagnitudetodevelopbetweenthethreeconductors.Itisthereforonlynecessarytoconsideroneconductorinfurthercalculations.Dependingonthedistancefromthepositionoftheshortcircuitfromthegenerator,hereitisnecessarytoconsidernear-to-generatorandfar-from-generatorshortcircuitsseparately.
Forfar-from-generatorandnear-to-generatorshortcircuits,theshortcircuitpathcanberepresentedbyameshdiagramwithACvoltagesource,reactancesXandresistancesR(Figure).Here,XandRreplaceallcomponentssuchascables,conductors,transformers,generatorsandmotors.
Fig.:
Equivalentcircuitoftheshortcircuitcurrentpathin
thepositive-sequencesystem
Thefollowingdifferentialequationcanbeusedtodescribetheshortcircuitprocess
wherewisthephaseangleatthepointintimeoftheshortcircuit.ThisassumethatthecurrentbeforeScloses(shortcircuit)iszero.Theinhomogeneousfirstorderdifferentialequationcanbesolvedbydeterminingthehomogeneoussolutionikandaparticularsolutioni²k.
Thehomogeneoussolution,withthetimeconstantg=L/R,solutionyields:
Fortheparticularsolution,weobtain:
Thetotalshortcircuitcurrentiscomposedofbothcomponents:
Thephaseangleoftheshortcircuitcurrent(shortcircuitangle)isthen,inaccordancewiththeaboveequation,
Forthefar-from-generatorshortcircuit,theshortcircuitcurrentisthereforemadeupofaconstantACperiodiccomponentandthedecayingDCaperiodiccomponent.Fromthesimplifiedcalculations,wecannowreachthefollowingconclusions:
●TheshortcircuitcurrentalwayshasadecayingDCaperiodiccomponentinadditiontothestationaryACperiodiccomponent.
●Themagnitudeoftheshortcircuitcurrentdependsontheoperatingangleofthecurrent.Itreachesamaximumatc=90(purelyinductiveload).Thiscaseservesasthebasisforfurthercalculations.
●.Theshortcircuitcurrentisalwaysinductive.
Methodsofshortcircuitcalculation
Theequivalentvoltagesourcewillbeintroducedhereastheonlyeffectivevoltageofthegeneratorsornetworkinputsforthecalculationofshortcircuitcurrents.Theinternalvoltagesofgeneratorsornetworkinputsareshortcircuited,andatthepositionoftheshortcircuit(faultposition)thevalue(isusedastheonlyeffectivevoltage(Figure).
●Thevoltagefactorc[5]considers(Table):
●Thedifferentvoltagevalues,dependingontimeandposition
●Thestepchangesofthetransformerswitch
●Thattheloadsandcapacitancesinthecalculationoftheequivalentvoltagesourcecanbeneglected
●Thesubtransientbehaviorofgeneratorsandmotors
●Thismethodassumesthefollowingconditions:
●Thepassiveloadsandconductorcapacitancescanbeneglected
●Thestepsettingofthetransformersdonothavetobeconsidered
●Theexcitationofthegeneratorsdonothavetobeconsidered
●Thetimeandpositiondependenceofthepreviousload(loadingstate)ofthenetworkdoesnothavetobeconsidered
Fig.:
Networkcircuitwithequivalentvoltagesource
a)three-phasenetwork,b)equivalentcircuitinpositivesequencesystem
Superpositionmethod
Thesuperpositionmethodisanexactmethodforthecalculationoftheshortcircuitcurrents.Themethodconsistsofthreesteps.Thevoltageratiosandtheloadingconditionofthenetworkmustbeknownbeforetheoccurrenceoftheshortcircuit.Inthefirststepthecurrents,voltagesandtheinternalvoltagesforsteady-stateoperationbeforeonsetoftheshortcircuitarecalculated(Figure).Thecalculationconsiderstheimpedances,powersupplyfeedersandnodeloadsoftheactiveelements.Inthesecondstepthevoltageappliedtothefaultlocationbeforetheoccurrenceoftheshortcircuitandthecurrentdistributionatthefaultlocationaredeterminedwithanegativesign(Figure).Thisvoltagesourceistheonlyvoltagesourceinthenetwork.Theinternalvoltagesareshort-circuited.Inthethirdstepbothconditionsaresuperimposed.Wethenobtainzerovoltageatthefaultlocation.Thesuperpositionofthecurrentsalsoleadstothevaluezero.Thedisadvantageofthismethodisthatthesteady-stateconditionmustbespecified.Thedataforthenetwork(effectiveandreactivepower,nodevoltagesandthestepsettingsofthetransformers)areoftendifficulttodetermine.Thequestionalsoarises,whichoperatingstateleadstothegreatestshortcircuitcurrent.Figureillustratestheprocedureforthesuperpositionmethod.
Fig.:
Principleofthesuperpositionmethod
a)undisturbedoperation,b)operatingvoltageatthefaultlocation,c)superpositionofa)andb)
Transientcalculation
Withthetransientmethodtheindividualoperatingequipmentand,asaresult,theentirenetworkarerepresentedbyasystemofdifferentialequations.Thecalculationisverytedious.Themethodwiththeequivalentvoltagesourceisasimplificationrelativetotheothermethods.Since1988,ithasbeenstandardizedinternationallyinIEC60909.Thecalculationisindependentofacurrentoperationalstate.Inthisbook,wewillthereforedealwithanddiscussthemethodwiththeequivalentvoltagesource.
Calculatingwithreferencevariables
Thereareseveralmethodsforperformingshortcircuitcalculationswithabsoluteandreferenceimpedancevalues.Afewaresummarizedhereandexamplesarecalculatedforcomparison.Todefinetherelativevalues,therearetwopossiblereferencevariables.
Forthecharacterizationofelectrotechnicalrelationshipswerequirethefourparameters:
●VoltageUinV
●CurrentIinA
●ImpedanceZinW
●ApparentpowerSin
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