Effect of water content on the conductivity of XLPE insulation

In HVDC cable insulation, the steady state electric field distribution is mainly determined by the following factors: DC conductivity, charge trapping, electron injection and ion formation. Factors which contribute strongly depend upon changes in morphology and chemical features within the insulation. In case of polymer insulated HVDC cables, nonuniform conductivity due to for example temperature gradients in the material will result in space charge formation and can yield an electric field distribution different from what will be expected in case of application of AC voltage. It is expected that the absorption of water will increase the formation of ions and thus increase the conductivity of XLPE insulation. The goal of this paper is to present results from conductivity measurements on XLPE cable insulation with varying water content. Measurements of charging and discharging currents were performed at applied electric stress in the range of 2-20 kV/mm, using Rogowski type test objects equipped with semiconducting polymeric electrodes. The measurements were carried out at temperatures of 40, 60 and 80 °C, at different relative humidities in the range from 10 % to 90 %. The test objects were conditioned in a climate chamber kept at the selected relative humidity and temperature of the subsequent measurement for up to 2 days prior to the conductivity measurements, thus ensuring that equilibrium water content was reached before the measurement started. In order to remove remnant space charge between each test, the test objects were kept grounded for a time 10 times longer than the high voltage charging time. The results show that an increase in water content increases the conductivity of the XLPE insulation. At the highest water content for a given temperature the conductivity was 1.8-3.4 times larger than at the lowest water content. The temperature- and field-dependency of the conductivity did not significantly change with increasing water content. The condu- tivity was found to increase proportionally to the square root of the water content, indicating dissociation of water as the origin of the increase.