Location of water tree degraded point along XLPE cable line using DC voltage

Until now many diagnosing techniques for water-tree degraded MV class XLPE cables have been proposed. However, although they can detect the average degree of degradation throughout the cable, the degraded part can hardly be located. The maintenance cost will be significantly reduced if the degraded part is located by repairing only the degraded part of the cable. This presentation will introduce a new method to locate the water-tree degraded part along a cable line by applying DC voltage. While DC leakage current measurement is carried out for diagnosing cables degraded by water-tree, it is often observed that the leakage current varies unstably. This phenomenon, called as "kicking", is considered to have a relation with partial discharges accompanied by water-tree. We tried to locate this highly degraded part along the cable length. Using a short size cable, it was proved that partial discharges, accompanied by an electrical tree generated from a water-tree, were taking place. The point was precisely located by using time-domain waveform measurement. Based on the above achievement, an equipment to locate highly degraded part along a cable line was developed. It was proved using a full-scale cable line that the equipment could locate the degraded part with a sufficient precision.