Degradation analysis of metal oxide varistors under harmonic distortion conditions

A thesis submitted in ful lment of the requirements for the degree Doctor of Philosophy in Electrical Engineering May 2016Modern electrical networks provide an opportunity for inevitable interaction between metal oxide arresters and power system harmonics. Therefore, these arrester devices are continuously exposed to the combined e ect of distorted system voltage and envi- ronmental thermal stresses. Recent studies supported by eld experiments have shown signi cant rise in the leakage current through these surge arrester devices when exposed to ac voltage with harmonics. However, the major shortcoming in the current knowledge and applications of varistor arresters resides on the reliability and the electrical stabil- ity of these overvoltage protection units, when subjected to long-term and continuous distorted ac voltage and thermal stresses from the environment. Commercially-sourced ZnO arresters of similar size and electrical properties are tested using standard ac accelerated degradation procedure or electro-thermal ageing test. The times to degradation, the coe cient of non-linearity, the reference voltages, as well as the clamping voltage measured are used to analyse the reliability and the electrical stability of the metal oxide-based arrester samples. The resistive component of the leakage current is extracted from the measured total leakage current. The three-parameter Weibull probability model is invoked in order to analyze the degradation phenomenon.MT201