The effects of electrical degradation on the microstructure of metal oxide varistor

In this paper, the findings from systematic experiments with the purpose to determine the degradation effect of single and multiple current pulses on the microstructure of the metal oxide (MO) varistor are described. Six distribution class varistors from one manufacturer were used in these experiments. The first part of the paper describes the electrical condition after application of single and multiple lightning current pulses. The results (before and after each elevated current impulse test) of 1 mA AC reference voltage and residual voltage are presented in this section. We have also investigated a new technique called "return voltage measurement" for monitoring of the degradation in MO varistors. This is described in detail. The second part deals with the microstructure observations of MO varistors. The results of microstructural examination of impulse current on MO varistors are examined in detail. This section also explains the relationship between the microstructural changes and electrical degradation of MO varistors