Thermally stimulated currents in haloed XLPE cable insulation

Direct buried distribution cables insulated with crosslinked polyethylene (XLPE) very often exhibit one or more halos. The halo is service induced and consists of microvoids filled with water. On viewing a cross section of the cable these microvoids will be found concentrated in concentric circular regions, hence the name halos. In order to examine the effects of the halo on the properties of the insulation, thin slices cut from the haloed and nonhaloed regions were prepared for thermally stimulated current measurements. The results indicated that material cut from the haloed regions contains a higher concentration of trapping sites than does material cut from halo free regions. Throughout the temperature range, −80 to +100°C the thermally stimulated current is greater for the haloed material. A sharp peak is observed at 30°C and a broader peak at around 0°C. The measurements are made on thin samples in a high vacuum. For this reason, it is unlikely that the microvoids were filled with water at the time of the measurements. These results are attributed to a defect introduced by the water intrusion, but not with the water itself.