Multistress endurance tests of extruded cable insulation

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Abstract
Many factors affect insulation life of an electrical system. The main factors producing polyethylene deterioration in in-service cables appear to be environmental moisture, partial discharges, operating stress and temperature. Laboratory experiments approaching service conditions have provided some knowledge about the effects of these factors studied separately, but combined application of more than two factors has rarely been undertaken. This paper describes techniques for and results from multistress endurance tests performed on extruded cable polyethylene o The method is applied to long-term breakdown tests on a number of samples of an insulation system. The four main factors that accompany cables in service are included. The endurance tests are designed on the basis of a statistical factorial design with every factor on two levels. The paper gives initial results of the set of long-term breakdown tests, extending for 2500 hours, performed on 2.25 and 1.6 mm thick polyethylene samples. The tests were carried out under 7, 10 and 12 kV/mm stress and 2 0 and 70° C temperatures on the samples with or without surface partial discharges and/or water. The times to breakdown were analyzed on the basis of the Weibull distribution0 It was found that interactions of partial discharges, water and temperature reduced greatly the time to breakdown and increased the shape parameter when compared with the same characteristics of samples aged only under temperature and electric stress.

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