Change of Varistor Voltage of Metal-oxide Varistor Influenced by Impulse Currents

Abstract

It was reported so far that degradation of varistor voltage of metal-oxide varistors (MOVs) was observed when short-tailed impulse currents such as 4/10µs or 8/20µs impulses were applied, whereas after application of impulse currents having rather long duration such as 2ms rectangular current, degradation of varistor voltage was not observed. In this study, it is found that degradation of varistor voltage occurs after application of 10/350µs impulse current having longer duration than short-tailed impulses. The charge transfer value per unit cross section area is found to be related to degradation of varistor voltages, though its effect is dependent on impulse current waveforms. The behaviour of degradation of varistor voltage inside of MOV elements stressed by short-tailed impulse currents is partially explained by non-uniformity of current distribution in the elements, which is confirmed by numerical electromagnetic analysis.