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.

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.