Time dependence of degradation in ZnO varistors

Abstract

Starting from the results of a previous paper on the functional relationship of the degradation of ZnO‐varistors and the influence factors (especially U, I,T) an equation system is derived to describe the time‐dependence of the degradation. The solutions including the contribution of degradation‐, recovery‐ and increasing effects of currents and temperature are given and discussed by special cases concerning the extent of the self‐heating and the electric operation conditions in certain approximations. Their validity is confirmed by characteristic experiments on the varistor performance.

Homogeneity of ZnO Varistors. Distribution of Local Electric Parameters before and after Degradation

“…Aging by electrical current is opposed by a recovery term, being dependent on degradation. This recovery term can very rapidly lead to saturation in the case of higher degradation rates (at constant temperature) [8].…”

The Degradation of Epoxy Resin‐Coated ZnO Varistors at Elevated Temperatures and Ambient Humidity Conditions

“…How to control or reduce the long-term degradation effect remains one of the major challenges for the investigators. In general, the degradation [1][2][3][4][5][6][7][8] refers to a gradual increase (steady or slow) in leakage current under the electric pulse stress or constant applied voltage. Many early investigators [9][10][11][12] had recommended some remedial steps such as heat-treatment (annealing) in conjunction with the adjustment in the varistor formulation and processing variables to control the degradation process for better stability of the device under applications.…”

“…That is why Eqs. (4) and (5) show different formats. It is assumed that the degradation mechanisms of the normal method and pressurised vapour method are identical.…”