Microstructure investigation of reference voltage and leakage current trends in varistor arresters

Recent literature in this study field shows that during electrical degradation process of Zinc Oxide (ZnO) varistors, the reference voltage (V 1mA) may show an increase or decrease trends. Decrease in the reference voltage following rise in leakage current are common symptoms of degradation or failure of these devices. However, increase in the reference voltage following drop in leakage current could also be experienced after exposure of varistor arresters to degradation agents. This work makes use of microstructure analysis in order to probe whether or not high reference voltage and low leakage current, experienced after continuous electro-thermal stress, constitute valid indication or symptoms of electrical degradation. Low voltage ZnO varistor samples (3 per temperature) are exposed to electro-thermal degradation test. The electro-thermal test is consisted of magnitude of 80% of voltage required to cause 1 mA leakage current through the arrester (0.8 V 1mA) and three point temperature 110°C,120°C and 135°C for a period of 48 hours. During the experiment, the leakage current is measured using an ammeter. Varistor voltage is also measured before and after degradation test. The experiment conducted in this work shows samples with decrease in the leakage current and increase in reference voltage (V1mA) trend under each temperature. SEM micrographs shows that even though the current was decreasing the samples has many holes as applied temperature increases. Energy dispersive X-ray spectroscopy (EDS) used to study the sample chemical compositions also shows the increase in the zinc oxide composition as the temperature increases