Effect of thermal aging on mechanical and physical performances of resisting water tree XLPE

Abstract

Cross-linked polyethylene(XLPE) insulating materials are widely used in the medium and high voltage cables because of their excellent electrical performances, as well as stable physical and chemical properties. With the long-term running of the XLPE cables, some degradation reactions of the insulating materials will happen under the action of long time thermal stress, which cause the irreversible changes for the chemical structure and morphology of these types insulating materials. The aging degradation will weaken the electrical properties of the cables and lead to insulation failure gradually, which brings about irreparable damage to the society and people. In this paper, both the resisting water tree and ordinary XLPE cable materials were selected as the study object, the aging temperature were set at 110°C, 130C, 150C to conduct accelerated aging experiments on the resisting water tree and ordinary XLPE cable materials, respectively. The effects of thermal aging on mechanical properties and electrical resistance of these XLPE insulation materials were studied, including the infrared transmission spectroscopy, the gel content, the tensile strength test and AC breakdown strength test and so on. The experimental results show that the tensile strength of the studied insulation materials increased firstly and then decreased at the selected aging temperatures. From 0 to 500 h of the aging time, the tensile strength of the two materials had a rising trend. The higher the aging temperature, the shorter the time to the turning point of the tensile strength. With the aging time prolonging, the tensile strength of both materials both decreased. And the tensile strength of the anti-water tree XLPE material was higher than that of ordinary XLPE. There exited significantly different with the absorption peak for both type XLPE materials in the 1000cm-1, which was due to the introduction of new functional groups to XLPE by the anti-water tree additive. As the aging temperature increased and the aging time was prolonged, the aging degree of ordinary XLPE materials was more severe than that of the antiwater tree XLPE insulation materials. The gel content of the two XLPE insulation materials increased firstly and then decreased with the increase of the thermal temperature and corresponding aging time, which shows that the cross-linking degree of the two XLPE increases first and then decreases. With the increase of the aging temperature and the aging time, the carbonyl index significantly increased for aged ordinary XLPE materials. However, the carbonyl index of the anti-water tree XLPE insulating materials changed smaller than that of the ordinary ones. Therefore, the anti-water tree XLPE insulating materials have better ability to inhibit thermal aging.