Influence of partial discharges on ZnO varistors

The varistors used in HV overvoltage limiters are constantly exposed to operating voltage and environmental hazards and also to transient hazards associated with, among others, overvoltages or pollution. Pollution exposures and the associated partial discharges (PD) are a major problem in the operation of overvoltage limiters. The discharges degrade the electrical properties of varistors which results in an increase in the conduction current active component below the varistor continuous operation voltage. This may lead to loss of thermal stability. To determine the effect of partial discharges on the varistor elements in HV overvoltage limiters, the service exposures were simulated on single varistors in laboratory conditions.