Abstract
AbstractThis paper describes the water tree diagnoses of 6.6‐kV class XLPE cables by the dc component method. The dc current component is measured by energizing the cable with ac high voltage, therefore, it would be suitable for on‐line diagnoses. The dc component is considered to be caused by the rectifying effect of water trees inside the insulation.In laboratory tests, the dc component showed a good correlation with the degradation. However, almost no correlation was seen in the on‐site measurement. The authors found that the cause of this bad correlation is the galvanic effect inside the jacket. This generates the noise current through the grounding circuit when the jacket resistivity is low. Based on this finding, the equivalent circuit of on‐site measurement was proposed.Considering the equivalent circuit, this paper proposes nine methods for discriminating the dc component due to water tree from noise current. Also described are the analytical results of each method. Furthermore, the on‐site or laboratory tests show that some of them are effective in separating the dc component due to water tree from noise current.
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