电气自动化专业外文文献及翻译短路电流电气类.docx

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