Effect of Bipolar Square Wave Voltage with Varied Frequencies on Electrical Tree Growth in Epoxy Resin

An electrical tree in epoxy resin induced by a bipolar square wave voltage at varied frequencies has been investigated. The morphology, growing characteristics, fractal dimension, and initiation proportion of anelectrical tree at 10 kV for a frequency ranging from 50 Hz to 20 kHz was studied. The results show that electrical trees are all branch-like in epoxy resin. It was revealed that there are three regions in the propagation of an electrical tree: 50 Hz~2 kHz (region A), 2~4 kHz (region B), and 4~20 kHz (region C). When imposed frequencies are increased from 50 Hz to 20 kHz, the number of tree branches, growing rate, and initiation proportion increase in region A; the number of tree branches and initiation proportion increase, while the growing rate decreases, in region B; and the growing rate and initiation proportion increase while the number of tree branches decrease in region C. The electrical tree length and width increase linearly with treeing time when frequencies were lower than 2 kHz, while increasing exponentially with treeing time when frequencies are higher than 4 kHz. The dielectric power loss of epoxy resin under varied frequencies is computed by an expansion in Fourier series, and the results indicate that the dielectric power loss increases with increasing frequency. The characteristics of an electrical tree are ascribed to a synergistic effect of the dielectric power loss of epoxy resin, space charge, and partial discharge under a bipolar square wave field.