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overvoltage_choppingpdf - XXIInd Int Symp on Discharges and...

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XXIInd Int. Symp. on Discharges and Electrical Insulation in Vacuum-Matsue-2006 Influence of Over-voltage Caused by Chopping Current on Electric Field Distribution in Vacuum Interrupter Gao Youhua' Liu Yanju2 Wang Erzhil Cao Yundong' Liu Xiaoming' 1. School of Electrical Engineering, Shenyang University of Technology, 58 Xinghua South Street, Tiexi District Shenyan, China, 110023 2. Scientific and Technological Department, Shenyang Ligong University, Hunnan New District, Shenyang, China, 110026 Abstract- A transient circuit model 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, which are regarded as boundary excitation. The differential equations of transient field are deduced, in which conductivity y is taken into account due to existence of plasma and metal gas between contacts, contact and shield. FEM is adopted to calculate the electric field in the vacuum interrupter and superposition of power frequency voltage and over-voltage of chopping current is regarded as boundary excitation, in which Delaunay method of mesh generation is introduced to discrete the solved region. Potential distribution curves at representative time are shown, which are compared with the one in the static field. I. INTRODUCTION Vacuum circuit breakers are widely used for their outstanding features, such as compactness, durability against frequent switching operations and reduction of maintenance work and so on. Even though new contact materials and structure of contacts ensure very low chopping current, the chopping current may still cause some faults in metalclad switchgears, short length bus and devices connected with bus [1][2]. A steep-fronted transient over-voltage caused by current chopping in the 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-voltage between stationary contact and movable contact may reach very high values due to a high di / dt value and conversion of magnetic into electric energy after the current is chopped [3-5]. Under the over-voltage, the electric field distribution in interrupter and insulated dielectrics will be different from static field. In this paper, a transient circuit model 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. The differential equations of transient field are deduced, in which conductivity y is taken into account.
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