About Establishing the Functional Service Limits for Low Voltage Metal Oxide Varistors

This paper describes an original testing method process designed to determine the stability envelope of a certain Metal Oxide Varistor (MOV) as a direct relation between the environmental temperature and the operational voltage in permanent operational mode. Although the current established throughout a MOV is heavily influenced by its own temperature, it is mandatory to find a certain relation between these two factors which are increasing the temperature of that power electronic device. Environmental temperature can increase the reference of its temperature and an additional voltage will cause an increased current generating a runaway heating process, having a direct connection between them. In order to avoid references to geometrical dimensions or voltage values, we will make all references to the load coefficient which is the ratio between the operational voltage (in DC) and the nominal opening voltage (also in DC). By means of experiments and a minimal mathematical model we will determine the equation of the critical load coefficient as function of the environmental temperature in case of disk varistors. This method could be useful for the design of more advance and reliable surge arrester based on existing Metal Oxide Varistors, for any voltage levels and any configurations.