电气工程及其自动化 外文翻译 外文文献 英文文献 短路电流Word文档下载推荐.docx

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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,theshortcircuitpowerisasuitablecharacteristicnum

ber.

●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.

1.2Shortcircuitpathinthepositive-sequencesystem

Forthesameexternalconductorvoltages,athree-poleshortcircuitallowsthreecurrentsofthesamemagnitudetodevelopbetweenthethreeconductors.Itisthereforonlynecessarytoconsideroneconductorinfurthercalculations.Dependingonthedistancefromthepositionoftheshortcircuitfromthegenerator,hereitisnecessarytoconsidernear-to-generatorandfar-from-generatorshortcircuitsseparately.

Forfar-from-generatorandnear-to-generatorshortcircuits,theshortcircuitpathcanberepresentedbyameshdiagramwithACvoltagesource,reactancesXandresistancesR（Figure1.2）.Here,XandRreplaceallcomponentssuchascables,conductors,transformers,generatorsandmotors.

Fig.1.2:

Equivalentcircuitoftheshortcircuitcurrentpathin

thepositive-sequencesystem

Thefollowingdifferentialequationcanbeusedtodescribetheshortcircuitprocess

wherewisthephaseangleatthe