Measurements of electrical resistances and investigations of factors affecting corrosion and breakages of shielding layers in extruded three-layer 6.6 kV XLPE cables

Corrosion and breakages of shielding layers are one of the factors for degradation of extruded three-layer (E-E type) 6.6 kV cross-linked polyethylene (XLPE) cables. Tohoku Electric Power Network has been measuring electrical resistances of shielding layers in E-E type 6.6 kV XLPE cables, and recently has found breakages of shielding layers. The authors performed an accelerated degradation test for shielding layers of E-E type 6.6 kV XLPE cables by taking account of aqueous solution, temperature, heat cycles, bending stress, frequency of an applied voltage, and water-proof layer in the test conditions, and investigated factors affecting corrosion and breakages of shielding layers. Corrosion, cracks, and breakages were found on shielding layers in the cables without water-proof layer, but no crack was found in the cables with water-proof layer, when heat cycles were applied to the cables, and shielding layers were supplied with artificial sea water. We investigated the suppressive effect of the cables with water-proof layer on cracks and breakages of shielding layers by measuring tensile strengths of copper tapes obtained from the shielding layers of the cables with and without water-proof layers.