Dielectric behaviour of hydrated and electrically stressed cross-linked polyethylene in a power distribution cable

Cross-linked polythene (XLPE) cables are being used increasingly for high voltage power distribution because of their enhanced mechanical properties over traditional pressure assisted dielectric cables^1,2. However, the expected service life of XLPE 15–36 kV distribution cables is yet to be achieved in practice. It has been suggested trial a premature failure of XLPE cables may be avoided by modifying the cable construction to prevent moisture ingress during operation. In general, water has very little effect on lyophobic organic hydrocarbons unless they contain impurities. However, there have been breakdowns of polythene and other hydrocarbon cables at low A.C. Stress under water due to the growth of ‘water trees’ for which there has been no definitive explanation as yet. The pressure of traces of salt, acids, alchohols or any substance which ionises in solution in water will be expected to worsen the effects of moisture. A reduction of electrical breakdown strength due to a pressure of moisture cannot be explained without a better-understanding of the dielectric polarisation process in such circumstances. With the above in view, we have undertaken in our laboratories a study of the electrical aging of XLPE distribution cables, immersed in water.