橋絡水トリーが発生した乾式架橋・三層同時押出方式6.6 kV CVケーブルの水トリー劣化診断に関する基礎検討

抄録

Harmful water trees, such as penetrated water trees bridging insulation, have not been found in removed dry-cured and extruded three-layer (E-E type) 6.6 kV cross-linked polyethylene (XLPE) cables since they were introduced into electric power systems more than 30 years ago. However, water-tree degradation becomes a serious problem as the operating duration increases, making it essential to diagnose water-tree degradation. Therefore, to establish the degradation index of water-tree degradation for these cables, growth of water trees should be accelerated for removed XLPE cables. In this study, an accelerated water-treeing test was performed for dry-cured and E-E type 6.6 kV XLPE cables removed after approximately 20 years of operation. As a result, we successfully found a penetrated water tree in one of the XLPE cables. Additionally, for the cable with a penetrated water tree, the values for electrical insulation characteristics, such as a DC leakage current, an AC superposition current, and dielectric loss tangent, were investigated. As a result, its DC leakage and AC superposition currents were above the threshold values indicating the existence of at least one penetrated water tree for E-T type 6.6 kV XLPE cables, whose type is older than E-E type. This result suggests that DC leakage and AC superposition currents are useful in finding penetrated water trees in water-tree degradation diagnosis for dry-cured and E-E type 6.6 kV XLPE cables.