Morphological and dielectric changes of XLPE insulation due to thermal aging

Transmission power cables are heated by conductor current. The morphology of the XLPE insulation may be changed by thermal aging. Disk-shaped insulating materials with semiconductive electrodes were used to understand this thermal effect against the dielectric breakdown strengths. The disk specimens were aged at 80 and 100°C before the breakdown strength measurements. These results showed that the dielectric breakdown strengths were improved with aging time. Analytical investigation of the insulating materials indicated that there were morphological changes such as increases of the lamella thickness and the spherulite diameter and decrease of the averaged lamella angle through heating. Cable specimens aged thermally with the same condition as the risk specimens also were observed with similar morphological changes in the insulation. Therefore it is concluded that the promotion of the crystallization by thermal aging is effective in improving the breakdown strength of the XLPE insulation unless it causes extreme deterioration.