Electrical Tree Characteristics of Epoxy Resin under Bipolar Square Wave Voltage

Epoxy resin is widely employed as insulating material in power electronic transformers (PET), a key device in smart grid systems. Bipolar square wave voltage is imposed on epoxy resin during operation, which may trigger electrical trees and breakdown of epoxy resin. Therefore, it is urgent to understand the electrical tree growth characteristics under square wave voltage in detail. The frequency and duty cycle of the applied bipolar square wave voltage were varied from 50 to 500 Hz and 30% to 50%, respectively. Accelerated electrical tree growth and denser branches were observed due to increased frequency and duty cycle. With frequency increased from 50 to 500 Hz, the length and width of the trees were found increased from 300 to 1013 μm and 195 to 1933 μm respectively. Many small branches were generated by increased frequency. These phenomena can be ascribed to synergistic effect of more frequent injectionextraction of charges and severer partial discharge under higher frequency. As a result, higher frequency and duty cycle can accelerate electrical tree growth, which cause severe damage even failure in epoxy resin. This study is beneficial for understanding the failure mechanism of epoxy resin due to electrical tree under bipolar square wave voltage.