Vol.:(0123456789) 1 3 Journal of Electrical Engineering & Technology (2021) 16:2875–2884 https://doi.org/10.1007/s42835-021-00819-0 ORIGINAL ARTICLE Ageing Characterisation of Varistor Arresters: A Statistical Model for Grain Response Under Applied Thermal Stress Lutendo Muremi 1  · Pitshou N. Bokoro 1 Received: 13 June 2020 / Revised: 23 September 2020 / Accepted: 3 June 2021 / Published online: 21 June 2021 © The Korean Institute of Electrical Engineers 2021 Abstract In this work, regression analysis is invoked to predict relationship between varistor microstructure response and tempera- ture for degradation characterisation or assessment of metal oxide surge arresters (MOSA). Accelerated degradation test is conducted on varistor samples—having similar electrical and physical characteristics—at randomly selected temperature points for equal period of time. The reference voltage—measured before and after accelerated degradation test—is used as the degradation criterion for varistor samples. Linear intercept performed on Scanning Electron Microscopy (SEM) micro- graphs is relied upon to estimate varistor average grain size at each applied thermal stress. The coefcients obtained for the developed regression model are statistically validated using the one-way analysis of variance (ANOVA). Results show that for each 1 °C variation in the accelerating temperature, which corresponds to 52. 6 h of service time, varistor average grain size increases by 0.143 microns. Keywords Accelerated degradation · Analysis of variance · Metal oxide varistor · Regression analysis · Scanning electron microscopy · Thermal stress · Varistor grain size 1 Introduction The popular use of metal oxide varistor (MOV)-based surge protection devices (SPDs) in power and/or telecom- munication networks, provides considerable justifcation for increased attention currently aforded to reliability analy- sis and/or design improvement of these transient protec- tion units under various operating conditions. Fundamen- tally, a MOV consists of the basic unit structure of MOSA, and consequently it is the most important component of MOSA devices. The life and performance of these gap- less arrester devices could be best understood in terms of the microstructure physics of MOV units [1–4]. The MOV microstructure is mainly composed of zinc oxide grains and intergranular regions [2, 3]. The primary objective of varistor arresters consists of clamping fast and slow-front transient overvoltages to safe voltage level, which is usually less than the basic insulation level (BIL) of the device under surge protection. While clamping surge voltages, varistor arresters should be capable to safely discharge the resulting high amplitude currents to earth or ground [5–8]. However, MOV-based arresters are prone to failure as a result of degra- dation. This phenomenon refers to irreversible change in the physical properties of MOV units (increase of average grain size, reduction of the double Schottky barrier, etc.), and con- sequently in the electrical properties (increased leakage cur- rent, change in the voltage/current (V - I ) curve, capacitance change, etc.) of these protective devices [9–11]. Continuous conduction of MOSA under thermal stress is reported to be one of the major causes of degradation [12–18]. Recent stud- ies in this feld show that no well-defned prediction model or correlation—relating microstructure response (average grain size) of MOSA to thermally-stimulated conduction (temperature)—has been proposed to date. The pertinence of such a prediction model lies in the fact that it may enable both manufacturers and consumers to characterise the extent of MOSA degradation for a determined service time. In this work, regression analysis is invoked to predict the relation- ship between varistor average grain size (dependent vari- able) and temperature (independent variable). Accelerated * Pitshou N. Bokoro pitshoub@uj.ac.za Lutendo Muremi lmuremi@uj.ac.za 1 Department of Electrical and Electronic Engineering Technology, University of Johannesburg, Johannesburg, South Africa