The impact of harmonics on the V − I characteristic of ZnO varistors

The continued presence of harmonic frequencies in modern power systems raises the fundamental question of suspected aggravation of ageing or degradation process of zinc oxide (ZnO) varistors under constant operating voltage mode. In this study, the long-term effect of power system harmonics on the ageing of ZnO varistor samples is analysed on the basis of the reference or the varistor voltage and the coefficient of non-linearity, which are measured from the voltage-current charactristic (V - I). The results obtained indicate that for an increase in the total harmonic distortion (THD) from 1.5 % to 8.47 % in the continuously applied voltage, at a constant temperature of 135°C, a decrease of 66.29 % in the reference voltage, and that of 76.92 % in the coefficient of non-linearity are observed in the varistor units exposed to higher content of harmonic-distortion.