Abstract
M.Tech. (Electrical Engineering)
MOVs are often used as overvoltage protective devices on electrical and electronic circuits as a result of their highly nonlinear V-I characteristic. Several studies conducted in this field have shown that the potential barrier developed in the microstructure between ZnO grains and intergranular regions are major factors contributing to nonlinear electrical conduction in varistor arresters. Performance reliability and stability of MOV under operation are entirely dependent on microstructure behaviour or condition. Therefore, degradation-based failure of varistor arresters resulting from continuous exposure of these devices to thermally-induced currents could be analysed in terms of changes in the potential barrier or the average size of ZnO grains. However, the relationship between changes in the average ZnO grain size and applied thermal stresses is not well defined in the literature. In this study, commercially-sourced low-voltage ZnO varistors of similar size were subjected to electro-thermal degradation as well as to SEM tests. The linear intercept method was used to determine the ZnO average grain size on the SEM micrographs. Linear regression analysis technique was also applied to model the relationship between applied thermal stresses and changes in the average grain size of ZnO. In order to assess the significance of the obtained model, ANOVA technique was used. Results obtained indicate that the MOV average grain size increases as the applied thermal stresses increases. The mean of average grain size was determined to be 5.78 μm, 7.25 μm, 9.19 μm, 14.61 μm and 17.35 μm on the observed thermal stresses. Therefore, an increase in the applied thermal stress has an effect on the microstructure changes. The obtained model shows that there is good correlation between the increase in the MOV average grain size and temperature.