Switching behavior of vacuum interrupters under practical system conditions

The vacuum interrupter is generally accepted as a very reliable circuit breaker in the medium-voltage power system. Therefore, this technology becomes increasingly important for further applications such as load breakers. Each new range of application results in different requirements on the contacts. For instance, copper-chrome in different compositions and fractions has been established as the best material for circuit-breaker applications in all distribution voltage levels. But the chopping currents may cause high switching overvoltages in case of inductive loads such as transformers. Concerning the measurement of chopping current, no standardized test circuits exist. In the literature, mostly synthetic test circuits are regarded, where the current is generated by discharging a capacitor. As the interrupter under test in this paper shall be optimized for load breaking, the used test circuit is adapted towards load breaking standards. The influences of the test circuit parameters are deeply examined in this paper. Current interruption at and before natural zero as well as the switching overvoltages for different small inductive currents (30 to 650 A) are evaluated. In addition, the influence of contact material (CuCr and Cu) is analyzed. From the result of this analysis and of further investigations recommendations on the applicability of these contacts shall be developed.