Analysis of Direct Current Integrated Charge in Cable Insulation with Electrical Tree

The electrical-tree degradation inside XLPE cable is a serious threat to the cables under operation. In this paper, the DCIC (direct current integrated charge) method was employed to evaluate the electrical tree inside XLPE insulation. During the preparation of cable samples, the tree inception voltage of ~8 kV was tested. Then 1, 5 and 10 needle electrodes were inserted into cable samples for the tree generation. After the preparation of samples, AC voltage of 10 kV was applied onto the needle electrodes for electrical tree growth. The treeing time was set to 0, 20, 40 and 60 min for the DCIC test. During the DCIC test, a DC voltage of 2 kV was applied on the conductor, and the integral capacitor was connected between the copper shield of cable and ground. From the experiment result, it can be found that with the increase of treeing time, the charging coefficient increases obviously if there are 5 and 10 electrical trees in the cable, this is maybe caused by the occurrence of polarization. The conduction current coefficient changes obviously before and after needles insertion. The information in DCIC curve can be used to analyze electrical tree degradation and mechanical damage in cables at the same time. This nondestructive testing method proposed in this paper will improve the testing efficiency and speed.