Combined Effect of Temperature and Humidity of Magnet-Wires on Partial Discharge Inception Voltage under Inverter-Surge Voltage

We investigated the combined effect of temperature and humidity on partial discharge inception voltage (PDIV) of magnet wires under repetitive surge-wave voltages. As the results, the PDIV increased as the temperature increased under uncontrolled humidity, which is peculiar under surge-wave voltage. On the other hand, the PDIV decreased lineally as the temperature increased under controlled humidity. From these results, we found that the PDIV increase under uncontrolled humidity was attributed to the effect of the relative humidity decrease corresponding to the temperature increase. Furthermore, the PDIV was also influenced by the moisture amount in insulation film. Under uncontrolled humidity, the PDIV during the temperature increase was lower than that during the temperature decrease with the hysteresis. We found two reasons for the PDIV hysteresis. One reason is the humidity increase in the air gap due to moisture evaporation from the film. Another reason is the electron emission from the film with the lower first ionization energy. Based on the above reasons, the generation probability of initial electrons during the temperature increase is higher than that during the temperature decrease, which decreases PDIV.