Research On The Thermal Characteristics Of ZnO Varistors

The core device of the Voltage Limiting type SPD is MOV, short circuited line and fire disaster are more and more frequent because of its damage caused by thermal effects, which severely harm safe and reliable operation of low-voltage distribution system. This dissertation analyzed factors which affect the thermal characteristics under AC/DC Voltage and the impact current, explored the mechanism why thermal damage occurred, and studied the relationship between the thermal properties and parameters by experiments.Comparison and analysis on the thermal stability test under AC/DC voltage, the results show that the withstand capacity of MOV is closely related to power loss, the energy absorption capacity and the heat dissipation capacity. The larger thermal capacity of MOV was, the slower temperature rise rate was and the larger withstand capacity was. The larger thermal resistance of MOV was, the worse heat dissipation capacity and withstand capacity was. The heat produced by power loss on MOV under DC Voltage is greater than that under AC Voltage, the main reason is that the voltage distribution ratio of MOV under DC Voltage is higher than that under AC Voltage.The MOV was tested under8/20μs impact current, and presented a simulation of temperature distribution of MOV using ANSYS finite elements software under impact current. The test result shows that the impact cycles and impact strength have a certain limitation, when the impact limit is approached, not only degradation degree of MOV is worse but also the surface temperature and residual voltage ratio have an increasing tendency; when the impact limit is reached, the properties can be maintained a certain value within a specific range. Thermal conductivity of grain inside the MOV can greatly accelerate the generation of thermal stress, hence the manufacture of MOV should minimize the thermal conductivity. The surface temperature of MOV under impact current was that high temperature region appeared in the area near entry point of current, then decreased gradually by the middle area to all around, and the minimum temperature appeared in the edges, which can provide a theoretical basis for design and manufacture of SPD’s tripping device.