Broad band dielectric investigation of cross-linked polyethylene use as electrical insulation on underground power cables

The control of the dielectric permittivity and losses of solid insulators, through an understanding of the origin of their dipole relaxation and ionic conduction processes, is essential for their use in electrical insulation. Dielectric Spectroscopy is a powerful tool in the investigation of dipolar and conduction in electrical insulating materials. In particular, the localisation of space charges at crystalline-amorphous interfaces in a semi-crystalline polymer, such as cross-linked polyethylene (XLPE), provides an internal field which could be unfavourable to the insulating properties of the polymer. On the obtained dielectric spectra, we can distinguish three different regions with different behaviours; a region between 10^-2 and 10^-1 Hz which characterises a phenomenon of polarization at the electrodes, between 0.1 and 100 Hz losses are independent of frequency, which may correspond to the DC conductivity, and a region between 100 Hz and 10 kHz characterized by a relaxation peak, moving towards higher frequencies, when the temperature increases, its an evidence of thermal stimulation of the relaxation phenomenon The dielectric permittivity presents a gradual decrease with increasing frequency and increasing temperature. This feature is masked by high index losses value due to DC conductivity.