Relation between the ac loss current method and the PEA method for water-treed length

Change of ac loss current through a non-penetrated water-treed XLPE sheet sample with applied voltage has been studied based on a model. In the model, the conductance of the water-filled channel layer is assumed to exhibit a nonlinear dependence on voltage. The observed changes of the magnitude and phase of the fundamental current and the third harmonic component can be well fitted by the model. It is found that the length of the water-filled channel layer approaches that of the water-treed region estimated by the PEA method at 2 kV stress. It is also found that the length of the water-filled channel layer obtained by the ac loss current method has a good correlation with the length of the water-treed region estimated by the PEA method.