Method to distinguish between space-charge and dipolar relaxation in the TSDC spectra of polyethylene electrical insulation

Medium-voltage cross-linked polyethylene (MV-XLPE) cables have an important role in the electrical power distribution system. For this reason, the study of XLPE insulation may lead to improve cable features and lifetime. Although relaxational analysis yield a lot of information about XLPE properties, sometimes their results are difficult to interpret. To overcome this handicap, we have used a combination of thermally stimulated depolarization currents (TSDC) and isothermal depolarization currents (IDC) techniques. In order to discard spurious effects from the semiconductor interfaces, preliminary measurements have been done on specially prepared cables. TSDC have been performed using conventional poling between 140°C and 40°C. IDC measurements also have been carried out at temperatures between 90°C and 110°C in 2°C steps. The TSDC spectra are dominated by a broad peak of uncertain origin. On the other hand, IDC show a combination of power and exponential currents. Exponential currents are fitted to a KWW model. The relaxation times obtained from the model present an Arrhenius behavior with E_a = 1.32 eV and τ₀ = 3.29×10^-16 s. The KWW parameter obtained is β = 0.8. The calculated depolarization current given by the exponential relaxation matches the predominant peak of TSDC spectra. Therefore, we conclude that in the MV cables studied the most visible peak of the TSDC spectrum has a dipolar origin.