Abstract
We have been operated ZnO surge arresters on our power systems since 1970's. But we have been anxious for 1st stage ZnO surge arresters, because these surge arresters have the thermal runaway behavior with the applied continuous operating voltage. I have been examined the resistive leakage currents of ZnO varistors which were enclosed in ZnO surge arresters installed in our substations 20 years ago. I estimated the resistive leakage currents with the long term ageing tests which were repeatedly consisted of supplied AC voltage periods and no voltage periods. As a result, changes of these leakage currents are caused by the migrations of interstitial zinc ions and oxygen ions. On this paper, I describe these ion migrations which are caused by supplied voltages or no supplied voltage and the cracks of depletion layers which could be caused by the migration of oxygen ions.
