Behavior of metal oxide based varistors subjected to partialdischarges in air

摘要

Experimental results are presented on the degradation of ZnOnvaristors under the action of partial discharges. The degradationnobserved is a corrosion of the insulating materials, giving rise to annincrease of the surface resistivity. The conducting deposit is probablynproduced by chemical reaction of the oxide surface with the nitric acidnformed by electrical discharge in humid air. In this acid, the strongncorrosion of uncoated varistors is more pronounced than in glass ornresin coated ones. Degradation is strongly influenced by the level ofndischarge energy. The increase in the surface conductivity could, innservice, give rise to high temperatures and consequently to the risk ofna thermal runaway and to a premature aging of varistors. It is concludednthat the quality of the insulating coating around the varistors used innsurge arresters for the protection of medium- and high-voltage powernlines should be improved