Mechanisms for Degradation of TR-XLPE Impulse Strength during Service Aging

While the retained ac strength of TR-XLPE distribution cable appears to be quite good at about 80% of the ac strength of degassed, as-manufactured cable, the impulse strength appears to drop linearly over time and has been reported to drop by as much as 50% during the first two years of normal service aging before leveling off. In this contribution, we discuss the mechanisms by which the impulse strength can degrade to a greater degree than the ac strength. Three frequency-dependent mechanisms have been identified, which include electro-thermal, electro-mechanical, and electrical.