It has been reported in polymer insulation that the ac loss current in phase to applied voltage is distorted with developing of water trees. We have obtained the distorted ac loss current from the water treed XLPE sheets. The observed wave forms were compared with the calculated ones by the model which takes account of a non-linear dependence of conduction current through the water treed region. It has been found that this model can explain qualitatively the changes of the relative magnitude of 3rd harmonic wave to the fundamental one and the phase of 3rd harmonic wave of ac loss current.
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