Investigations on ageing of low density polyethylene in power frequency AC fields

Abstract

Insulation in electrical equipment is a very crucial aspect while designing any electrical system especially in case of high voltages in any part of a power system. Hence it is crucial to determine the properties and performance of an insulating material. The experiments deal with the measurement of breakdown voltages and determination of ageing of an insulating material under accelerated AC fields. The insulating material used in our experiments is Low Density Polyethylene (LDPE) as it is an extensively used insulator for power cables. LDPE makes an excellent electrical insulator because of its high resistivity and good tracking resistance. While insulation is essential for designing any electrical system, its failure often leads to the subsequent failure of the system. Every insulating material undergoes degradation with time even if it is subject only to operating or working stress. The test method used, to relate the stress and time till breakdown, in our experiments is steppedstress testing. This type of tests gaining momentum due to their certainty of completion of ageing experiments compared to constant-stress testing where incomplete data leads to censoreddata analysis. Also the times consumed can be low depending on the choice of voltage step size and time step duration. The variations were in terms of voltage levels and the time of exposure of the specimen to a particular voltage level. The distribution of breakdown voltages for the specimen for different step sizes can be studied using Weibull distribution. Inverse Power Law and Miner’s Theory of cumulative damage gives a quantitative insight of the breakdown phenomena in insulators and can be used to estimate the equivalent life of the material under test. Using these statistical methods, the ageing and expected performance of the insulating material can be obtained.