A consideration of changes of AC breakdown voltages during an accelerated test of immersed dry-cured XLPE cables

The authors describe the results of an accelerated aging test of 6.6 kV cross-linked polyethylene (XLPE) insulated cables made by a dry-curing process. The accelerated aging test described permits the study of the AC breakdown mechanism. The accelerated test was carried out by applying 6.9 kV, at 1 kHz, at room temperature, with the conductor strands and the insulation shield kept constantly moist. The AC breakdown voltage was investigated for a given aging time. Within three months of aging time, the breakdown voltage decreased to about half of the initial breakdown voltage. This decrease is found to be mainly caused by the moisture permeating the insulation of the cables. To clarify the influence of moisture on the breakdown voltage, some further tests were carried out. The results clearly show that the breakdown voltage is reduced in the presence of even an infinitesimal moisture content. Also, as expected, the presence of micro-water paths in the amorphous region caused the reduction of breakdown voltage