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Ageing study of ZnO-based varistors in transformer oil

In this paper the AC ageing of ZnO-based varistors in transformer oil is studied by an accelerated ageing during 6000 hours. Three kinds of varistors have been studied: 1-non-coated, 2-coated by resin and 3-coated by glass . As well the thermal behaviour of varistors has been studied at different temperatures and voltages. It is found that the coating and environmental conditions have a great influence on the thermal behaviour and the lifetime of varistors. The oil improves their thermal behaviour. As well it improves the I(V) characteristics of the coated varistors while for the naked ones it causes a premature degradation. It is seen that glass could be, from the thermal point of view, a good material for coating the varistors.