Micro-Analytical Method of Submicron Insulation Morphology near the Semiconducting Interface in an XLPE Cable to Evaluate its Dielectric Properties

A new evaluation method of submicron morphological changes of the insulating polymer near the semiconducting interface in an XLPE power cable was proposed based on TEM observations. The method provides higher sensitivity on material degradation due to aging than the conventional methods. A long term aging test was carried out on an XLPE cable specimen with insulation thickness of 9mm. The specimen was aged under AC voltage stress of 63kV at 1kHz and heat-cycling thermal stress by the conductor current with real time of 13, 164h. The experimental results show that the dielectric breakdown strengths did not decrease or rather slightly increased in spite of generation of bow-tie trees after the aging. Material analysis of the insulation was done and the result indicated that there were submicron morphological changes such as spherulite diameter and lamella thickness. It was suggested that these changes were brought by the thermal stress and might cause the slight increase of the breakdown strengths. Therefore it may be concluded that the new method is useful to detect fine material changes.