Deterioration trend on electrical treeing of underground cable insulation

This study discussed the relationship between the electric tree growth and partial discharge characteristics in the cross-linked polyethylene (XLPE), and proposed the evaluation rule of insulation status. The partial discharge was tested by cutting the cable insulation XLPE in specific depth, to make a 4mm clearance between the pinpoint and the semi-conducting layer. A fixed voltage was applied until destruction. The voltage was 14, 15, 16 and 17 kV respectively. Meanwhile, the computer captured the partial discharge signals regularly and processed them into phase analysis data. According to the analysis, the discharge parameters of various tests were highly similar, and the discharge number was more suitable to be the characteristic value of insulation diagnosis. The characteristic curve and differential characteristic slope of characteristic values of various tests could be obtained using regression analysis. The confidence intervals were calculated according to the characteristic slope turning points of all tests as the thresholds of initial term, final term and midterm of insulation. This is the insulation status evaluation rule proposed in this paper.