Field-aged cable material diagnosis by thermally stimulated currents

The Thermally Stimulated Currents (TSC) technique has been studied as a method for detecting dry state water trees in field-aged cables. Measurements were taken on medium voltage extruded cable samples of crosslinked polyethylene (XLPE). Samples were peeled-off from several well characterized field-aged and one unaged cables. Insulating material has been characterized with regard to water-tree density. TSC peaks were observed around -30°C (β) and 110°C (α) for field-aged and unaged insulation. Low-temperature peak intensity variations throughout cable radii have been observed and assumed to be related to the cable insulation characteristics. No correlation has been observed between the total integrated charge from β peak integration and the dry state water-tree surface density of the samples. In addition, TSC and TSPC (Thermally Stimulated Polarization Currents) measurements on treed and untreed regions of the same cable at the same radius have been carried out in order to compare samples having the same characteristics except for the presence of dry state water trees. Variations in α peak intensity was related to the lower water-tree resistivity even in the dry state