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Unformatted text preview: XXIIndInt.Symp.onDischargesandElectricalInsulationinVacuum-Matsue-2006 Influence of Over-voltage Caused b y C h o p p i n g Current on Electric Field Distribution in Vacuum Interrupter GaoYouhua' LiuYanju2 Wang Erzhil CaoYundong' LiuXiaoming' 1.SchoolofElectricalEngineering,ShenyangUniversityofTechnology,58XinghuaSouthStreet, TiexiDistrict Shenyan,China,110023 2.ScientificandTechnologicalDepartment,ShenyangLigongUniversity,HunnanNew District, Shenyang,China,110026 Abstract- A transient circuitmodel forthevacuum circuit breaker switching three-phase inductive motor is established and the steep-fronted over-voltages waves caused by chopping current are calculated. Discrete FourierTransform (DFT)isintroducedtodecompound the step-front transient over-voltage. Several main frequency components areobtained,which areregardedasboundary excitation. The differentialequations oftransientfieldare deduced, in which conductivity y is taken into account duetoexistenceofplasmaand metalgasbetween contacts, contactand shield.FEM isadoptedtocalculatetheelectric fieldinthevacuum interrupterand superpositionofpower frequencyvoltage and over-voltage ofchopping currentis regarded as boundary excitation, in which Delaunay method of mesh generation isintroduced to discrete the solved region. Potential distribution curves at representativetime are shown, which are compared with theoneinthestaticfield. I.INTRODUCTION Vacuum circuit breakers are widely used for their outstanding features, such as compactness, durability againstfrequent switching operations andreduction of maintenanceworkandsoon. Even thoughnew contact materials and structure of contacts ensure very low chopping current,thechopping currentmay stillcause some faultsinmetalclad switchgears, shortlengthbus anddevicesconnectedwithbus. A steep-fronted transientover-voltagecausedbycurrentchoppinginthe vacuum interrupter has already been an attentive problem to researchers. It may affect the source insulation (transformer windings), the load insulation (induction motor windings) and vacuum interrupter insulation.The over-voltagebetween stationarycontact andmovablecontactmayreachveryhighvaluesduetoa high di/dtvalue and conversion of magnetic into electricenergyafterthecurrentischopped[3-5]. Undertheover-voltage,theelectricfielddistribution ininterrupterandinsulateddielectricswillbedifferent fromstaticfield.Inthispaper,atransientcircuitmodel for the vacuum circuit breaker switching three-phase inductive motor is established and the steep-fronted over-voltages waves caused by chopping current are calculated. Discrete Fourier Transform (DFT) is introduced to decompound the step-front transient over-voltage. Several main frequency components are obtained.Thedifferentialequationsoftransientfieldare deduced,inwhichconductivity y istakenintoaccount....
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