Ageing of metal oxide varistors due to surges

Surge arresters are exposed to a series of stresses that trigger changes in their nonlinear behaviors. This paper deals with the ageing of metal oxide varistors concerning the stresses associated to high current short duration surges. The ageing of the samples is evaluated by analyzing the evolution of the resistive component of the leakage current after every impulse current application in non energized samples. This procedure aims at determining the ageing prescribed by impulse currents disregarding the superposition of power frequency voltage effects. To this a sequence of 8 × 20 µs impulses were applied to sets of metal oxide resistors at different temperatures. After every impulse application and the resistor reached the room temperature the value of the resistive component of the leakage current and the power losses were recorded and compared. These obtained data are statistically analyzed and the results are correlations between the ageing evolution and number of applied impulses, the influence of the temperature and discharge current amplitude. The set correlations are compared to studies from 80's, and concerning the provisions of relevant standards.