Return voltage measurements to investigate the degradation of zinc oxide varistor

Abstract

Due to their reliability and accuracy, many modern diagnostics based on dielectric voltage response, such as polarization/depolarization current (PDC), voltage decay (VD) and return voltage (RV) measurements, have been used in monitoring ageing processes of metal oxide (MO) varistors. Among these diagnostics, recently, RV measurement (RVM) seems to be an increasingly popular method as it has high sensitivity to the condition of varistors and low sensitivity to disturbances in vicinity of the field measurements. Nonetheless, the basic interpretation based on the RVM essential parameters – peak RV, time-to-peak RV and initial slope of RV - provides insufficient information of the MO varistors condition since they are inevitably dependent on the measuring parameters such as the charging and discharging times as well as the test object temperature. Hence, this project focuses on a new way in interpreting the RVM parameters based on dielectric time constants analysis using an equivalent circuit of varistor microstructure, namely the Maxwell-Model. In order to investigate the ageing processes of MO varistors, two types of accelerated degradation techniques – impulse and heat degradations – are systematically conducted on test samples. Experimental results are presented and discussed in detail according to the underlying physical mechanism. On the basis of this concept, a sensible ageing parameter, p-factor, is used for better characterization of the ageing status of varistors.