Unveiling frequency dependence of electrical treeing degradation in epoxy resin under bipolar square wave voltage

The epoxy insulation used in high frequency transformers encountered breakdown easily due to bipolar square wave voltage with steep rising/falling edge and high frequency. An insight into the frequency dependence of electrical tree development in epoxy resin under bipolar square wave voltage was elucidated in this paper. It was found that luminescence existed in all treeing channels, where the intensity of luminescence at the electrical tree tip under a high frequency (>4 kHz) was stronger than that at a low frequency (<4 kHz), indicating an electric field delivery from the needle tip to the channel tip at high frequency. Correspondingly, the fluorescent material deposited on the wall of the tree channel evolved from discontinuous at a low frequency to continuous at a high frequency with aggravated degradation. In addition, the frequency dependence of electrical tree breakdown related to field delivery was revealed. By combining the field delivery along treeing channels and the frequency dependence of the mechanical properties of epoxy resin, the growing model of an electrical tree under bipolar square wave voltage was illustrated elaborately. It was proposed that the dominant factors that drove electrical tree propagation were electric field force at a low frequency and partial discharge at a high frequency, respectively. This work can boost the advancement of polymers, electrical equipment, and power electronics at a high frequency.