Electrical conductivity of medium voltage XLPE insulated cables

Improved knowledge on the charge transport phenomena in XLPE is important to control the field distribution in the cable insulation, and thus further improve the reliability of future HVDC cables. Polarization and depolarization currents was measured on two medium voltage cables at 40, 60, and 80 degrees C with an applied average electric field varying from 2.5 to 19 kV/mm. The conductivity of the insulation was calculated based on the measured quasi steady state current. The two cables were identical apart from the XLPE material (95mm(2) Cu conductor, 3.4 mm insulation). The field dependence of the conductivity was found to be non-linear for the entire field range. Measured currents were fit to the Poole-Frenkel and hopping conduction. The fit to Poole-Frenkel gave a relative permittivity of 2.9, while the fit to hopping conduction gave an average hopping distance of 19 nm. The best fit to the measurements was obtained with Poole-Frenkel conductivity. The temperature dependence follows an Arrhenius relation with activation energy of approximately 1.4-1.6 eV.