Water treeing in thermally preoxidized cross-linked polyethene cable insulation

The authors report results of water treeing tests on the insulation of unaged, cross-linked polyethene cable which had been thermally preoxidized to levels typical of those measured in field-aged cable insulations after several years of service. The tests showed that moderate levels of oxidation present in the insulation prior to voltage aging have no significant effect on initiation and growth of vented water trees. However, preoxidized specimens grew fewer and shorter bow-tie trees than nonpreoxidized specimens. Insulation tested at a lower electric stress exhibited larger retention of the initial breakdown strength after aging than the insulation tested at a higher stress even though the lengths of the water trees were comparable for both insulations. For all preoxidation conditions water treeing caused some depletion of the antioxidant from the insulation. Nonpreoxidized specimens did not show any excess of oxidation with time of electrical aging even after development of very heavy treeing. However, preoxidized samples did show increasing oxidation inside water treed regions as compared with electrically unaged specimens