Development of on-site diagnostic method for XLPE cable by harmonics in AC loss current

Water tree degradation in XLPE cables used for under-ground distribution or transmission lines is a serious problem. Some diagnostic methods to detect water tree degradation have been developed, but the sensitivity of them is not sufficient for 22 kV/66 kV class XLPE cables. The reason is that conventional methods are based on detecting signals from water trees bridged over insulation layer. In case of 22 kV/66 kV class cables, even unbridged water trees will be harmful because of their higher operating stress, so that there are demands on highly reliable new diagnostic methods for such class cables. We found that water tree in XLPE insulation generates harmonics in the AC loss current because of the non-linear voltage-current characteristics of itself. And, we found that the harmonics in AC loss current are closely correlated to water tree growth, break down strength, and the other aspects of the degradation. We made the on-site diagnostic system based on those results, and have applied to the actual 66 kV XLPE cable lines more than 22. As the result, we confirmed that it was suitable for the diagnostic method for 22 kV/66 kV class cables.