Capacitance switching with vacuum circuit breakers-a comparative evaluation

A statistical approach for determining the probability of reignition during capacitance switching by vacuum circuit breakers is described, wherein the dielectric recovery following current interruption is probed by overstressing the contact gap. It is found that the breakdown data can be quite well described by the Weibull distribution. The method is used to compare the capacitance switching performance of vacuum interrupters having copper/chromium and copper/bismuth contacts. Results imply a reignition rate for copper/bismuth almost two orders of magnitude higher than that for copper/chromium. Reignitions tend to occur when the arc angle (the period of arcing before current zero) is short, regardless of the contact material. The gap between CuCr contacts recovers more quickly than that between CuBi contacts. Very few reignitions after 2.5 ms were observed with CuCr contacts, whereas for CuBi reignitions persisted from 6 to 7 ms. This increases the likelihood of an overvoltage accompanying a reignition.<
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