I. Introduction
Modern electrical power networks nowadays rely on power electronics components for high-speed switching and control of several devices for various purposes [1], [2]. This practice, although efficient and reliable, has proven to be culprit of introducing harmonic current and voltage components in power systems [3], [4]. Although the negative influence of power system harmonics on leakage current-based condition monitoring of ZnO varistors have been extensively discussed in recent studies [5] – [8], the suspected aggravation or accelerated ageing of ZnO varistors as a result of continuously applied distorted ac voltage has not been looked at. In this study, commercially-sourced ZnO varistor samples are tested using accelerated electro-thermal ageing tests with controlled injection of harmonic frequencies. The characteristic curve, which provides valuable information (reference voltage, coefficient of non-linearity…) relative to the electrical stability as well as the ageing status of ZnO varistors [9], [10], is measured before and after accelerated ageing tests. The results obtained suggest that high content of harmonic-distortion in the continuosly applied voltage contribute to the reduction of the refence voltage and the coefficient of non-linearity, which both prompt the shift of the characteristic curve towards the high conduction region. This therefore confirms the electrical breakdown or ageing of ZnO varistor devices.