Dry-cured and extruded three-layer (E-E type) 6.6kV cross-linked polyethylene (XLPE) cables was developed to reduce water tree degradation, and they were introduced into electric power systems more than 30 years ago. Three kinds of insulation thickness are used at this voltage class depending on the conductor size of XLPE cables, and it is possible that the insulation thickness affects residual AC breakdown voltage and degradation diagnostic data. Also, water immersion into XLPE is one of important factors for water tree degradation, and it could affect those electrical insulation characteristics. In this study, the authors measured AC breakdown voltages for two kinds of insulation thickness of dry-cured and E-E type 6.6kV XLPE cables removed after 12 to 30 years of operation and accelerated water-tree degraded XLPE cables. As a result, it was found that the AC breakdown voltages for the smaller insulation thickness were smaller than those for the larger insulation thickness, and the AC breakdown voltages with water immersion were smaller than those without water immersion. Taking account of these data, effects of the insulation thickness and water immersion on the lifetime of this type of cable were discussed. It was also found that effect of insulation thickness on DC leakage current and AC superposition current was negligible. To detect harmful water trees before they bridge cable insulation, we applied the current-integrated method to some of the cables, where water tree degradation was accelerated under an AC voltage at 3.8kV and 1000Hz and under several thermal conditions including the heat-cycles between room temperature and 60°C, for measuring DC leakage currents. As a result, it was found that two types of degradation degree due to non-bridged water trees could be distinguished using the current-integration method, but it was difficult using the conventional one.
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