Contact Heating under Back-to-Back and Single Capacitor Inrush Current Arcs

The restrike probability of vacuum interrupters during capacitive switching is in a wide sense related to the inrush peak current during energizing of a capacitor. However, a simple relation between contact erosion, melting and solidification during making and the rupture of a weld and restrike probability during breaking is difficult to prove because of the random occurrence of restrikes. Fact is that the appearance of the inrush current arc depends on the peak inrush current and frequency. Typical values range from several kA at 400 Hz for single capacitor banks up to 20 kA at 4000 Hz for back-to-back capacitors. The current density of the inrush current arc can be estimated from the light emitted by the arc recorded by a high-speed camera. It is observed that for the low frequency arcs at single bank switching, the arc spreads out with the speed of cathode spots. For the high frequency arcs at back-to-back capacitor switching, the arc column expands with much higher speeds. With the time dependent current density of the arc and a reasonable arc voltage, giving the arc power density, the temperature of the contact surface at the point of pre-ignition can be calculated. Though the obtained temperatures are in the same range, their temporal development at back-to-back switching differs considerably from that at single capacitor switching.