電氣工程及其自動(dòng)化 外文翻譯 外文文獻(xiàn) 英文文獻(xiàn) 短路電流

Short-circuitcurrent1TermsandDefinitionsThefollowingtermsanddefinitionscorrespondlargelytothosedefinedinIEC60909.Refertothisstandardforalltermsnotusedinthisbook.Thetermsshortcircuitandgroundfaultdescribefaultsintheisolationofoperationalequipmentwhichoccurwhenlivepartsareshuntedoutasaresult.Causes:1.Overtemperaturesduetoexcessivelyhighovercurrents.2.Disruptivedischargesduetoovervoltages.3.Arcingduetomoisturetogetherwithimpureair,especiallyoninsulators.1.Interruptionofpowersupply.2.Destructionofsystemcomponents.3.Developmentofunacceptablemechanicalandthermalstressesinelectricaloperationalequipment.circuit:AccordingtoIEC60909,ashortcircuitistheaccidentalorintentionalconductiveconnectionthrougharelativelylowresistanceorimpedancebetweentwoormorepointsofacircuitwhicharenormallyatdifferentpotentials.circuitcurrent:AccordingtoIEC60909,ashortcircuitcurrentresultsfromashortcircuitinanelectricalnetwork.Itisnecessarytodifferentiateherebetweentheshortcircuitcurrentatthepositionoftheshortcircuitandthetransferredshortcircuitcurrentsinthenetworkbranches.Initialsymmetricalshortcircuitcurrent:Thisistheeffectivevalueofthesymmetricalshortcircuitcurrentatthemomentatwhichtheshortcircuitarises,whentheshortcircuitimpedancehasitsvaluefromthetimezero.Initialsymmetricalshortcircuitapparentpower:Theshortcircuitpowerrepresentsafictitiousparameter.Duringtheplanningofnetworks,theshortcircuitpowerisasuitablecharacteristic

number.shortcircuitcurrent:Thelargestpossiblemomentaryvalueoftheshortcircuitoccurring.stateshortcircuitcurrent:Effectivevalueoftheinitialsymmetricalshortcircuitcurrentremainingafterthedecayofalltransientphenomena.aperiodiccomponent:Averagevalueoftheupperandlowerenvelopecurveoftheshortcircuitcurrent,whichslowlydecaystozero.breakingcurrent:Effectivevalueoftheshortcircuitcurrentwhichflowsthroughthecontactswitchatthetimeofthefirstcontactseparation.voltagesource:Thevoltageatthepositionoftheshortcircuit,whichistransferredtothepositive-sequencesystemastheonlyeffectivevoltageandisusedforthecalculationoftheshortcircuitcurrents.method:Thesuperpositionmethodconsidersthepreviousloadofthenetworkbeforetheoccurrenceoftheshortcircuit.Itisnecessarytoknowtheloadflowandthesettingofthetransformerstepswitch.factor:RatiobetweentheequivalentvoltagesourceandthenetworkvoltageUn,dividedby3.electricalcircuit:Modelforthedescriptionofthenetworkbyanequivalentcircuit.shortcircuit:ThevalueofthesymmetricalACperiodiccomponentremainsessentiallyconstant.shortcircuit:ThevalueofthesymmetricalACperiodiccomponentdoesnotremainconstant.Thesynchronousmachinefirstdeliversaninitialsymmetricalshortcircuitcurrentwhichislargerthantwicetheratedcurrentofthesynchronousmachine.shortcircuitimpedance:

Theimpedanceofthepositive-sequencesystemasseenfromthepositionoftheshortcircuit.shortcircuitimpedance:Theimpedanceofthenegative-sequencesystemasseenfromthepositionoftheshortcircuit.shortcircuitimpedanceTheimpedanceofthezero-sequencesystemasseenfromthepositionoftheshortcircuit.Threetimesthevalueoftheneutralpointtogroundimpedanceoccurshere.circuitimpedance:Impedancerequiredforcalculationoftheshortcircuitcurrentsatthepositionoftheshortcircuit.p1.2Shortcircuitpathinthepositive-sequencesystemForthesameexternalconductorvoltages,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:Equivalentcircuitoftheshortcircuitcurrentpathinthepositive-sequencesystemThefollowingdifferentialequationcanbeusedtodescribetheshortcircuit

processwherewisthephaseangleatthepointintimeoftheshortcircuit.ThisassumethatthecurrentbeforeScloses(shortcircuit)iszero.Theinhomogeneousfirstorderdifferentialequationcanbesolvedbydeterminingthehomogeneoussolutionikandaparticularsolutioni2k.Thehomogeneoussolution,withthetimeconstantg=L/R,solutionyields:Fortheparticularsolution,weobtain:Thetotalshortcircuitcurrentiscomposedofbothcomponents:Thephaseangleoftheshortcircuitcurrent(shortcircuitangle)isthen,inaccordancewiththeaboveequation,Forthefar-from-generatorshortcircuit,theshortcircuitcurrentisthereforemadeupofaconstantACperiodiccomponentandthedecayingDCaperiodiccomponent.Fromthesimplifiedcalculations,wecannowreachthefollowingconclusions:ThecircuitcurrentalwayshasadecayingDCaperiodiccomponentinadditiontothestationaryACperiodiccomponent.Themagnitudeoftheshortcircuitcurrentdependsontheoperatingangleofthecurrent.Itreachesamaximumatc=90(purelyinductiveload).Thiscaseservesasthebasisforfurther

calculations.shortcircuitcurrentisalwaysinductive.1.4MethodsofshortcircuitcalculationTheequivalentvoltagesourcewillbeintroducedhereastheonlyeffectivevoltageofthegeneratorsornetworkinputsforthecalculationofshortcircuitcurrents.Theinternalvoltagesofgeneratorsornetworkinputsareshortcircuited,andatthepositionoftheshortcircuit(faultposition)thevalue(isusedastheonlyeffectivevoltage(Figure1.4).

Thevoltagefactorc[5]considers(Table1.1):Thedifferentvoltagevalues,dependingontimeandpositionThestepchangesofthetransformerswitchThattheloadsandcapacitancesinthecalculationoftheequivalentvoltagesourcecanbeneglectedThesubtransientbehaviorofgeneratorsandmotorsThismethodassumesthefollowingconditions:ThepassiveloadsandconductorcapacitancescanbeneglectedThestepsettingofthetransformersdonothavetobeconsideredTheexcitationofthegeneratorsdonothavetobeconsideredThetimeandpositiondependenceofthepreviousload(loadingstate)ofthenetworkdoesnothavetobeconsideredFig.1.4:Networkcircuitwithequivalentvoltagesourcea)three-phasenetwork,b)equivalentcircuitinpositivesequencesystem

1.4.2SuperpositionmethodThesuperpositionmethodisanexactmethodforthecalculationoftheshortcircuitcurrents.Themethodconsistsofthreesteps.Thevoltageratiosandtheloadingconditionofthenetworkmustbeknownbeforetheoccurrenceoftheshortcircuit.Inthefirststepthecurrents,voltagesandtheinternalvoltagesforsteady-stateoperationbeforeonsetoftheshortcircuitarecalculated(Figure1.5b).Thecalculationconsiderstheimpedances,powersupplyfeedersandnodeloadsoftheactiveelements.Inthesecondstepthevoltageappliedtothefaultlocationbeforetheoccurrenceoftheshortcircuitandthecurrentdistributionatthefaultlocationaredeterminedwithanegativesign(Figure1.5c).Thisvoltagesourceistheonlyvoltagesourceinthenetwork.Theinternalvoltagesareshort-circuited.Inthethirdstepbothconditionsaresuperimposed.Wethenobtainzerovoltageatthefaultlocation.Thesuperpositionofthecurrentsalsoleadstothevaluezero.Thedisadvantageofthismethodisthatthesteady-stateconditionmustbespecified.Thedataforthenetwork(effectiveandreactivepower,nodevoltagesandthestepsettingsofthetransformers)areoftendifficulttodetermine.Thequestionalsoarises,whichoperatingstateleadstothegreatestshortcircuitcurrent.Figure1.5illustratestheprocedureforthesuperpositionmethod.

Fig.1.5:Principleofthesuperpositionmethoda)undisturbedoperation,b)operatingvoltageatthefaultlocation,c)superpositionofa)andb)1.4.3TransientcalculationWiththetransientmethodtheindividualoperatingequipmentand,asaresult,theentirenetworkarerepresentedbyasystemofdifferentialequations.Thecalculationisverytedious.Themethodwiththeequivalentvoltagesourceisasimplificationrelativetotheothermethods.Since1988,ithasbeenstandardizedinternationallyinIEC60909.Thecalculationisindependentofacurrentoperationalstate.Inthisbook,wewillthereforedealwithanddiscussthemethodwiththeequivalentvoltagesource.1.5CalculatingwithreferencevariablesThereareseveralmethodsforperformingshortcircuitcalculationswithabsoluteandreferenceimpedancevalues.Afewaresummarizedhereandexamplesarecalculatedforcomparison.Todefinetherelativevalues,therearetwopossiblereferencevariables.Forthecharacterizationofelectrotechnicalrelationshipswerequirethefour

parameters:VoltageUinVIinAImpedanceinWApparentpowerSinVA.Threemethodscanbeusedtocalculatetheshortcircuitcurrent:1.TheOhmsystem:Units:kV,kA,V,MVA2.Thepusystem:Thismethodisusedpredominantlyforelectricalmachines;allfourparametersu,i,zandsaregivenasperunit(unit=1).Thereferencevalueis100MVA.ThetworeferencevariablesforthissystemareUBandSB.Example:ThereactancesofasynchronousmachineXd,X2dgiveninpuorin%pu,multipliedby100%.3.The%/MVAsystem:Thissystemisespeciallywellsuitedforthefastdeterminationofshortcircuitimpedances.Asformalunitonlythe%symbolisadd.

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短路電流術(shù)語(yǔ)和義以下術(shù)語(yǔ)和定義對(duì)應(yīng)IEC標(biāo)準(zhǔn)60909未出現(xiàn)在本書中的術(shù)語(yǔ)可以在該標(biāo)準(zhǔn)中查詢。短路和接地故障主要是操作設(shè)備的帶電部分被分流而導(dǎo)致絕緣損壞的結(jié)果。因1.溫度過高導(dǎo)致強(qiáng)烈的過電流；2.火花放電導(dǎo)致過電壓3.由于水分和污穢空氣混雜導(dǎo)致的電弧作用，特別是在絕緣體上。果：1.供電中斷2.系統(tǒng)部件癱瘓3.在電氣操作設(shè)備中產(chǎn)生不可接受的機(jī)械力和熱應(yīng)力。路：根據(jù)IEC60909，短路是經(jīng)歷一段相對(duì)低電阻或在個(gè)或更多不同電位之間的電阻間意外或故意的導(dǎo)電連接。路電流：根據(jù)IEC60909，短路電流是在電力網(wǎng)絡(luò)中短路的果。在這里有必要區(qū)分在短路過程中產(chǎn)生的短路電流和在網(wǎng)絡(luò)分支中的轉(zhuǎn)移電流。始對(duì)稱短路電流：這是在短路出現(xiàn)瞬間，短路阻抗從零開始變化時(shí)的對(duì)稱短路電流的有效值。始對(duì)稱短路視在功率：短路功率代表了一個(gè)虛構(gòu)的參數(shù)。在網(wǎng)絡(luò)規(guī)劃中，短路功率是一個(gè)合適的典型參數(shù)。最大短路電流:短路時(shí)可能的最大電流瞬時(shí)值。穩(wěn)態(tài)短路電流：初始對(duì)稱短路電流在暫態(tài)過程中衰減完畢之后的電流有效值。短路電流非周期分量：慢慢衰減的短路電流上下包絡(luò)曲線的平均值。稱斷路電流：聯(lián)絡(luò)開關(guān)第一次接觸分離時(shí)流過短路電流的有效值。效電壓源：

被轉(zhuǎn)移到正序系統(tǒng)作為唯一有效的短路位置的電壓，并且主要用于短路電流的計(jì)算。加方法：疊加法考慮到在發(fā)生短路前網(wǎng)絡(luò)的負(fù)荷情況。因此很必要知道負(fù)荷留了和變壓器開關(guān)的設(shè)定。壓因素：等效電壓源和網(wǎng)絡(luò)電壓之間除以三的比例。效電路：網(wǎng)絡(luò)描述的模型采用等效電路。離發(fā)電機(jī)短路：對(duì)稱交流周期分量維持原本不變的量的短路形式。近發(fā)電機(jī)短路：對(duì)稱交流周期分量不保持不變的值的短路。同步機(jī)首先會(huì)產(chǎn)生一個(gè)大于兩倍額定電流的初始對(duì)稱短路電流。序短路阻抗：短路位置正序系統(tǒng)的短路阻抗。序短路阻抗：短路位置負(fù)序系統(tǒng)的短路阻抗。序短路阻抗：短路位置零序系統(tǒng)的短路阻抗。就是中性點(diǎn)到短路位置阻抗的三倍。路阻抗：計(jì)算短路位置的短路電流所需要的阻抗。1.2

正序系的短路路徑對(duì)于相同的外部導(dǎo)體電壓，三相短路允許同一數(shù)量級(jí)的三相電流在三相導(dǎo)體中發(fā)展所以在進(jìn)一步計(jì)算中只需要考慮一相導(dǎo)體的情況根據(jù)短路位置到發(fā)電機(jī)的距離，這里有必要將遠(yuǎn)離發(fā)電機(jī)短路和靠近發(fā)電機(jī)短路這兩種情況分開考慮對(duì)于遠(yuǎn)離和靠近發(fā)電機(jī)短路的情況短路路徑可以用一個(gè)有交流電壓源電抗X電阻R構(gòu)成的網(wǎng)絡(luò)圖表示）這里和R代所有的原件，如電纜，導(dǎo)體，變壓器，發(fā)電機(jī)和電機(jī)。

RkXk

ibikU圖1.2短路電流路徑在正序系統(tǒng)中的等效電路下面的微分方程可以用來描述短路過程+=,(1.1)是短路點(diǎn)的相位角。這是假設(shè)電流在S關(guān)閉（短路）之前是零。非線性一階微分方程可以通過決定齊次解ik和特解i2k求解。=+齊次解有一個(gè)時(shí)間常量,方程式為：=對(duì)于特解，我們得出：=總短路電流由兩部分構(gòu)成：=

(1.2)(1.3)(1.4)(1.5)根據(jù)以上方程，單相短路電流的短路角為：==(1.6)對(duì)于遠(yuǎn)離發(fā)電機(jī)形式的短路，短路電流是由一個(gè)不變的交流周期分量和一個(gè)衰減的直流非周期分量構(gòu)成。從簡(jiǎn)化計(jì)算，我們可以得出以下結(jié)論短路電流總是由一個(gè)固定的交流周期分量和一個(gè)衰減的直流非周期分量構(gòu)成；

短路電流的大小取決于電流的工作角，最大值9（純電感負(fù)載）。這種情況作為進(jìn)一步計(jì)算的基礎(chǔ)。短路電流都是感應(yīng)的。1.4

短路電計(jì)算方法:三相系統(tǒng)中的短路電流有三種計(jì)算方法：1.故障位置計(jì)算等效電壓源；2.加法確定負(fù)載流量情況；3.態(tài)計(jì)算。1.4.1

等效電源:這里的等效電壓源主要作為發(fā)電機(jī)或投入電網(wǎng)的短路電流計(jì)算的唯一有效電源。發(fā)電機(jī)和投入電網(wǎng)的內(nèi)部電壓是短路的，短路地點(diǎn)（故障位置）的值就作為唯一有