Abstract
This paper describes the water tree diagnoses of 6.6 kV class XLPE cables by DC component method. This method measures the DC current component by energizing the cable with AC high voltage, therefore, it would be suitable for on-line diagnoses. The DC component is considered to appear due to the rectifying effect of water trees inside the insulation.
In the laboratory test, 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 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, two methods were proposed to discriminate the true DC component out of noise current. One is to by-pass the noise current from the measurement apparatus, and the other is to statistically estimate the true DC component by using the transition in the jacket resistivity. The on-site test were carryed out to confirm the applicability of these methods.
