Abstract
The influence of DC bias on electrical tree growth under 13 kV square-wave voltages at 50 Hz in epoxy resin is investigated. Tree growth is captured in a needle-plane configuration using a CCD camera and digital microscope. Tree growth is accelerated under positive unipolar square waves in comparison with a bipolar square waves and negative square waves. Tree growth under positive and negative unipolar square wave voltages can be described as a three-stage process, while under bipolar square wave voltages tree growth follows a five-stage process. Reverse tree growth is observed before breakdown only under bipolar square waves. No reverse tree growth was seen under unipolar square waves of either polarity. Damage was observed at the bottom of the plane surface of epoxy where the reverse tree channels initiated.
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