Efeito das correntes de impulso de curta duração na degradação e na capacidade de absorção de energia de varistores a ZnO

Abstract

Installed surge arresters are subjected to different types of overvoltages, and the varistors, which are their active element, must be able to not only absorb the energy from these overvoltages, but also present degradation levels that do not compromise its functionality after the event. The lack of correlations between the energy absorbed by a varistor and its degradation motivated the investigation proposed in this research, in which varistors from different manufacturers had their energy absorption capabilities evaluated by current impulses at different waveforms with durations in the order of microseconds. The degradation level of the samples during the ageing process was evaluated through the measurement of the leakage current between impulse applications, in which the power, the resistive peaks and the harmonic distortion were analyzed. The results present a direct relation between the impulse duration and the number of discharges with the same energy withstood in different waveforms, and indicate that the time under and overvoltage is more critical to varistor ageing than the peak value of the discharged current. However, samples submitted to shorter impulses presented more intense variations on their leakage current, which can lead to a thermal runaway of the varistor in operating conditions.