Electrical tree propagation in epoxy resin under bipolar square wave field with varied frequencies

Epoxy resin, used as the insulation material of solid state transformer, has to endure the harsh operating conditions of bipolar square wave voltage with high frequency. It is necessary to study the behavior of electrical tree induced by divergent field under bipolar square wave field. In this work, the morphology, growing properties, fractal dimension and breakdown characteristics of electrical tree in epoxy resin under bipolar square wave field with varied frequency was investigated. The voltage frequencies changed from 4 kHz to 20 kHz at 10 kV and 50% duty cycle. The results showed that electrical trees were all branch-like in the experiment. The electrical tree grew rapidly with treeing time after a period with slow propagation and accelerated by higher frequency. Furthermore, the growing speed was increased while number of branches decreased when frequency increased. There existed a maximum fractal dimension when frequency increased from 4 kHz to 20 kHz, which is ascribed to the synergistic effect of polarity reversal and decay of space charge produced by high voltage needle tip and partial discharge. It is suggested that carbon substance was deposited on the channel wall of electrical tree, which was revealed by Raman spectroscopy. The field-driven tree growth (FDTG) model was utilized to explain the development of darker and wider channel originating from ground to needle tip.