Physico-chemical analysis of a failed cable

Studies have been conducted of a section of crosslinked polyethylene insulation from an 80-kV cable that had failed under field aging. The cable had been deliberately treated to excessive conductor temperatures for a long period of time. Both steady and cyclic conditioning had been imposed. The cable sections showed multiple melting peaks in differential scanning calorimetry studies, dependent on location relative to the conductor, which are interpreted in terms of the the known laboratory aging behavior. The section nearest the conductor showed evidence of degradation, increased void content, and the presence of water trees, together with the most complicated melting curves. All the evidence demonstrates that significant chemical and physical degradation occurs within the insulation close to the conductor during combined thermal and electrical aging under moderate conditions