| Summary: | This study analyses a gas insulated switchgear explosion accident caused by a failure of the circuit breaker (CB) in a thermal power plant, which resulted in the explosion of the CB and the burning of the potential transformer (PT). Through X-ray detection and disassemble examination of the damaged CB, it is found that the A-phase moveable contact was not opened completely. After analysing the voltage waveform obtained from the phasor measurement unit recorder system, it is speculated that this failure is caused by the process of repeated arc strike. For verification and further analysis, the fault process is simulated in the EMTP; the arc is properly modelled by an extended KEMA's equation-based model which is proposed in this study. The simulation results match well with the actual situation; the contact gap would be repeated breakdown because its breakdown voltage is reduced, and the very fast transient overvoltage (VFTO) would be generated in these repeated strike processes. Due to the steep front and high amplitude of VFTO, the insulation of the PT would be damaged and led to the occurrence of PT short-circuit fault. The current would increase rapidly and causes the CB to explode subsequently.
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