Ageing of underground power cables: species migration from semi-conductive layers to the insulating layer

Medium voltage underground cables were aged in air at temperatures between 70°C to 140°C. The changes of the structural properties of the crosslinked polyethylene (XLPE) insulating layer were characterized by physico-chemical techniques: Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FTIR). The performed analyses revealed chemical ageing (oxidation, chemical consumption of additives, post-crosslinking...) and physical ageing (migration of chemical species). The accelerated thermal tests on stripped XLPE samples showed, as expected, a mild oxidative degradation. In contrast, ageing tests conducted in the presence of the semi-conductive layers revealed a striking difference, with a migration of extra carbonyl species into the XLPE insulation.