Electrical Tree Propagation in Epoxy Resin under Superimposed Sinusoidal and Repetitive Pulse Waveforms for Converter-fed Motor Insulation

The ground-wall insulation (epoxy/mica insulation) of form-wound converter-fed rotating machines is subjected to pulse width modulation (PWM) waveforms during operation. In this study, the behavior of electrical trees in epoxy resin, a major cause of the insulation degradation, is experimentally investigated under simulated operating waveforms, i.e. superimposed sinusoidal and repetitive pulse waveforms. Tree length, propagation speed, and morphology are evaluated through microscope observations. The results show that the tree propagation speed becomes significantly faster with an increase in pulse voltage ratio, compared to the case of the AC waveform alone. Considering the observation of the tree morphology, it is inferred that the tree channel density under the superimposed waveforms becomes sparser than that under the sinusoidal alone, resulting in the electric field enhancement at the tip of the tree channels and the faster propagation.