Comparison of XLPE Cable Peelings with Different Electro-Thermal Histories using Endurance Test and Residual Charge Methods

In a previous programme sections of the same XLPE-insulated ac-cables were stressed under different electro-thermal conditions and for various durations. Here, thin peelings from the cable insulation have their electrical endurance compared with peelings taken from an unstressed cable section, under the same test conditions. The aim is to verify whether or not the cable stressing had altered the inherent endurance capability of the insulation material i.e. aged it, and to identify markers for the dominating process/es that degraded the materials. The contributions from defects and screening layers have been excluded from this study. The endurance test was set at very high ac field and high temperature to maximise the stress and minimise the life of the material. All samples were conditioned before the test to remove any humidity and residual cross-linking by-products. A set of four samples was used for each material and the breakdown times were analysed by Weibull statistics. The results clearly show that only sample sets from cables that included a thermal stressing component display a reduction in the inherent lifetime, compared to the unstressed reference. The residual charge after the endurance test was measured by the pulse-electro-acoustic (PEA) technique for all the failed samples as well as the suspended ones. Two types of behaviour were observed but it was found that the amount of retained charge was not related to the endurance life of the material