Behavior of metal oxide based varistors subjected to partial discharges in air

Experimental results are presented on the degradation of ZnO varistors under the action of partial discharges. The degradation observed is a corrosion of the insulating materials, giving rise to an increase of the surface resistivity. The conducting deposit is probably produced by chemical reaction of the oxide surface with the nitric acid formed by electrical discharge in humid air. In this acid, the strong corrosion of uncoated varistors is more pronounced than in glass or resin coated ones. Degradation is strongly influenced by the level of discharge energy. The increase in the surface conductivity could, in service, give rise to high temperatures and consequently to the risk of a thermal runaway and to a premature aging of varistors. It is concluded that the quality of the insulating coating around the varistors used in surge arresters for the protection of medium- and high-voltage power lines should be improved