Application of the Thermal Step Method to characterize the charge ability of XLPE insulation with and without ethylacrylate copolymer

In Belgium, two types of high voltage underground power cables with different compositions have been installed, namely XLPE (an homopolymer cross linked polyethylene) and XLPE-EA copolymerized with ethylacrylate. The presence of space charge is now acknowledged to have a main role on electrical stresses applied to the insulation, and consequently in insulation lifetime. Part of the study was devoted to obtain a better characterization of their capacity of this two installed insulations to store space charges under AC. in this paper, results of space charge measurements by the Thermal Step Method (TSM) and Thermally Stimulated Depolarization Currents (TSDC) of cable pieces after the manufacturing, are presented. During alternative poling (65 kV RMS : 13 kV/mm RMS) of a 15 kV cable, only the XLPE cable stocks space charge after ten weeks. No charge has been observed in the XLPE-EA insulation after six months with the TSM technique. Some measurements have been carried out with a Scanning Electron Microscope (SEM) used in mirror mode. The bulk sample analysis by TSM and SEM used in mirror mode results are compared and conclusions are given for the XLPE-EA insulation in comparison with the XLPE. These results show differences between the two XLPE cables, and an evolution of the physical structure of the polyethylene insulation is highlighted.