Insulation characteristics of dry-cured and extruded three-layer (E-E type) 6.6 kV removed XLPE cables with additional accelerated water-tree degradation

摘要

An accelerated water-treeing test was performed for dry-cured and extruded three-layer (E-E type) 6.6 kV XLPE cables removed after approximately 20 years of operation. Additionally, the values for electrical insulation characteristics, such as DC leakage current, dielectric loss tangent, residual charge, and AC superposition current at 1 Hz, were investigated. As a result, it was found that nearly all these values increased after the accelerated water-treeing test. Furthermore, it was found that an AC superposition current was above that of an E-T type 6.6 kV XLPE cable having penetrated water trees when a DC leakage current was above a threshold value of 0.1 pA, indicating the existence of at least one penetrated water tree for E-T type 6.6 kV XLPE cables. Also, breakdown tests were done for some of the cables after the accelerated water-treeing test, and water trees were observed.