Electrical tree characteristics under AC and repetitive pulse voltages in wind turbine generator composite insulation

The composite insulation system in a wind turbine generator (WTG) is subjected to either the AC or the repetitive pulse voltage, which may lead to degradation in the insulation. In this paper, the effects of voltage amplitude, waveform, repetition rate and composite insulating material constitution on the electrical tree morphology and development are studied. The results indicate that higher AC voltage produces sparser branches, shorter initiation time, and higher growth rates in the impregnating varnish and along interfaces. Electrical trees are initiated and grow easier under repetitive pulse voltage than AC voltage. Increasing repetition rate of pulse voltage promotes the initiation and propagation of electrical trees. It has been found that the peak of the pulse overshoot on the rising edge plays an important role in the initiation and propagation of electrical tree, while the average of pulse voltage dominates the breakdown time of the composite insulation system.