Dependence of the chopping current level of a vacuum interrupter on parallel capacitance

The chopping current level of a vacuum interrupter is the mean amplitude of current before the natural current zero at which the arc extinguishes. The distribution of chopping currents of a vacuum circuit breaker using CuCr25 contact material was measured in a laboratory circuit consisting of a 3-phase 20 kV cable system and a dry-type distribution transformer rated 900 kVA. The cable length between the upstream feeding transformer and the circuit breaker was several hundreds of meters. Downstream, the cable length to the transformer was approximately 5 m. An inductive load connected to the distribution transformer provided a 50 Hz current of 10 A rms through the circuit breaker. The chopping current level was investigated under these conditions as a function of arcing time. More than 25 breaking operations performed at each arcing time, between 2 ms and 8 ms, showed no significant dependence on the arcing time. The mean chopping current level was around 3 A. After addition of surge capacitors of 130 nF between the transformer terminals and ground, the chopping current level increased to 6 A–8 A. The arc current amplitude started to oscillate before the current chopped with a frequency depending on the size of the surge capacitor. This is further analyzed in order to explain the different chopping current levels with and without parallel capacitance.