The Effect of Morphology on the Impulse Voltage Breakdown in XLPE Cable Insulation

This paper describes fundamental studies into the effects of (1) the thermal properties, and (2) the morphology of cross-linked polyethylene on its impulse voltage breakdown stress, as determined by the use of miniature cables. Nine grades of high pressure polyethylene, characterized by molecular structure, are used in the studies. The degree of crystallinity of XLPE is found to be closely related to impulse voltage breakdown stress, which increases when crystallinity is increased. Therefore, the fact that impulse voltage breakdown stress decreases with temperature can be associated with a decrease in crystallinity. Likewise the thickness of the amorphous zone is found to increase with temperature, although the crystal thickness has changed onlyslightly. From this point of view, the amorphous zone appears to play an important role in the occurrence of impulse breakdown in that the breakdown seems to originate from the amorphous zone.