Melting Pattern of Vacuum Interrupter Contacts Subjected to High Inrush Currents at High Frequencies

One important aspect in capacitor switching concerns the high inrush current during the energizing of the capacitor. For vacuum interrupters, it is assumed that the modification of the contact surface due to high inrush currents has the highest impact on the current breaking performance. One peculiarity of capacitor switching is the high rate of switching operations easily achieving 1000 operations in a couple of years. Two vacuum breakers were therefore operated up to 1000 times in a simple test circuit simulating the making of high inrush currents. According to standards, peak and frequency of inrush currents are 20 kA and 4250 Hz for back-to-back capacitor switching, or around 5 kA and 400 Hz for single bank switching. The range of inrush currents was extended here to peak currents up to 60 kA and frequencies up to 24 kHz. Peak current and frequency have an important impact on radius and current density of the inrush current arc, and thus on the energy deposited under the arc roots. Under the conditions prevailing here, the whole surface of the contacts exhibits a uniform melting pattern with the depth of the solidified melt being less than 1 mm after 1000 operations. It is assumed that the arc moves on the contact surface driven by the radial magnetic forces generated by this kind of contact and distributes the arc energy all over the contact surface. The inrush current integral (
ICI
) might be a measure of the available energy for contact melting or erosion. However, since the
ICI
has no relation to the arc current density, it is only representative for a particular melting pattern of the contact surface if frequency and peak inrush currents are varied in a limited range.