Location and examination of defects in XLPE cable insulation

After breakdown testing of a cable it is difficult to identify the cause of the breakdown, because the insulation in the region of breakdown is usually heavily damaged. The authors describe an approach in which the insulation from cables extruded under normal conditions is peeled to form continuous lengths of thin tape. Results from AC breakdown tests of these tapes and microscopic and elemental analysis of detected weak sites are presented. The density, size, and radial position of the sites were measured. The results indicate that this technique can be used to find explanations of the lower breakdown stress of cable insulation compared to the strength of insulating material measured under laboratory conditions. The number of weak insulation sites in 12-kV XLPE (cross-linked polyethylene) cables was found to vary across the insulation wall, with the highest density in a region close to the conductor screen