Numerical simulation of the residual charge of vacuum interrupters

The amount of charge carriers remaining in the switching gap of the vacuum interrupter (VI) at current zero, called residual charge (RC), is simulated numerically. Corresponding rate equations of the arc plasma were solved in combination with the development of anode surface temperature and metal vapor emission. The arc plasma is assumed to consist of "fast ions" and "slow ions." Production of "slow" ions results from charge exchange between ions and vapor atoms. At low arcing loads, the "fast" ion component prevails and RC depends only on the current decay rate at current zero. Density of "slow" ions, and consequently, RC, increases drastically when melting of the contacts and metal vapor emission occurs. The numerical results, i.e., the RC in dependence of arcing load as well as the time-behavior during free recovery, are in considerable agreement with experiments. Application of synthetic circuit testing is compared to direct testing of vacuum interrupters.