New residual-charge measurement for water-tree deteriorated power cable

We have been investigating the residual-charge measurement as a diagnostic method for water-tree deteriorated cross-linked polyethylene cables. The conventional method uses the amount of the residual charge as a diagnostic index, but it has a difficulty in estimating the degree of water-tree deterioration in a long line, because the amount of the residual charge depends on the length of the cable. The distribution of residual charges obtained by using the improved method we proposed here could correlate to the water-tree distribution in insulation of the cable. In this method, the residual-charge component released by higher AC voltage application, which could correspond to longer water trees, is used as an index how degree the insulation is deteriorated by water trees. This index is independent of the length of the cable. Therefore, the method we have developed is effective to diagnose the degree of water-tree deterioration. We applied this method to the cable line in fields and confirmed its effectiveness.