Dependence of the Chopping Current Level of a Vacuum Interrupter on Parallel Capacitance

The distribution of chopping currents of a vacuum circuit breaker using CuCr25 contact material was measured in a laboratory circuit consisting of a three-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. An inductive load connected to the distribution transformer provided a 50-Hz current of 10 A rms. Under these conditions, the mean chopping current level was between 3.2 and 3.5 A and showed no significant dependence on the arcing time, which was varied between 2 and 8 ms. After addition of a surge capacitor of 130 nF between the transformer terminals and ground, which very much reduces the overvoltage during switching, the chopping current level increased to 5.6 to 7.7 A in dependence of the arcing time. In order to explain this, the interaction of the arc with the electric circuit in particular with the cables on both sides of the breaker has to be evaluated. If the cables are long, travelling waves need to be considered as well as all return current paths in a three-phase circuit. The coincidence of excited network oscillations with random arc instabilities may result in either momentary amplification or damping of such oscillations. An additionally installed capacitor serves as a source sustaining a momentary current reduction after an arc instability and allows the excitation of a subsequent arc instability leading to an increase of the chopping current with longer arcing times. With low capacitance on the load side, there is no interaction and therefore no influence from the arcing time.