The life of XLPE cable insulation containing water trees

Abstract

The most common polymers used as insulation in high voltage power cables, cross-linked polyethylene (XLPE), low density polyethylene (LDPE), and ethylene propylene rubber (EPR), degrade with time when exposed to moisture and electrical stress. Tree-like growths, known as water trees, appear in the insulation which lower the breakdown strength and cause failures in service. Although there have been extensive studies, the exact mechanisms of the degradation have not been clarified and a valid accelerated aging test has yet to be developed. A good review of the subject has recently been made by Shaw and Shaw [1]. One technique to increase the resistance to water treeing is to incorporate a tree retardant additive in the polymer. This paper will describe the results of tests to evaluate the treeing resistance of miniature cables insulated with conventional XLPE (ST-XLPE) and a tree retardant XLPE (TR-XLPE). Both cables were steam-cured. The effects of heating the cables to 90°C or 130°C prior to electrical aging at room temperature were also examined. Also, tests were performed using laboratory-molded slab specimens.