Space charge in XLPE near the electrode interfaces

Electric field and space charge profiles were measured in 110 μm thick planar XLPE samples machined from the insulation of power transmission cables. The samples carried vacuum-evaporated aluminium electrodes, and were poled at field-strengths around 50 kV/mm (both polarities) for periods up to 48 hr. The electric field and space charge profiles were measured using the LIMM technique. The experimental data were analysed using a combination of the polynomial regularization method (PRM) and the L-curve method (LCM). The profiles were recorded immediately after the end of poling, and after the sample had been grounded for various times. The spatial resolution of the profiles in the thickness direction was 1-2 μm. Negative space charge densities up to 400 C m^-3 and negative field strengths up to 20 kV/mm were recorded at distances 1-2 μm from the cathode immediately after poling. Within similar distances of the anode the corresponding figures were +300 to -100 C m ^-3 and -5 to +5 kV/mm. In one sample the space charge profile decayed rapidly with time in short circuit, but in two others the decay rate was negligible.