Effect of thermal ageing on charge dynamics and material properties of 320 kV HVDC XLPE

This paper reports the effect of thermal ageing on the space charge characteristics, DC conductivities and trap energy distributions of 320 kV HVDC XLPE. Specimens were prepared by a heating press method in the laboratory. The specimens were placed in a thermal oven and thermally aged at 130 °C up to 30 days. Space charges were measured by a pulsed electro-acoustic setup. The results showed that the reference samples had limited hetero charge accumulations near both electrodes in the samples. The homo-charge accumulation was appeared with an increase of the aged time. The space charge accumulation in the samples was transferred from hetero-charge accumulation to homo-charge accumulation with an increase of the ageing time. The electric field distribution in the sample gradually increased with the appearance of the homo-charge accumulation, whereas the homo charges evidently distorted the electric field in the samples with the increasing ageing time. The DC conductivities of the aged samples consistently increased with the increase of the measured electric field. The obtained results of apparent mobility of the charge carriers, the activation energy and the trap energy density illustrated that thermal ageing could facilitate the homo-charge accumulation in the sample. The dominant effect of thermal ageing on the sample was elucidated by using various analytical techniques. It was found that the thermal ageing would affect the material properties and facilitate the increase of the trap energy density and the space charge accumulations in the samples.