Surge ageing of metal oxide varistors

The objective of this paper is to analyze the ageing of metal oxide varistors (MOV) due to surges by the changes of the nonlinear behavior. The surge arrester installed in the field is submitted to the lightning stress superposed to the power frequency voltage, and so, submitted to an ageing process due to both cumulative stresses. Therefore, this procedure looks for the ageing associated only to lightning impulse currents, in other words, without the power frequency voltage contribution. To get it, sequences of 8/20 μs impulse currents were applied to sets of MOV, considering different amplitudes and varistors temperatures. The base for the analysis is the comparison of the leakage current and power losses values - both for a fixed reference voltage - before and after each impulse application on non energized samples. The obtained values were statistically analyzed to be the basis for correlations to the ageing process, the number of applied impulses, the influence of the temperature and the discharge current amplitude. A comparison to studies from 80´s is presented, and some details are commented about the requirements of relevant standards.