Conductivity properties of XLPE insulation used for HVDC cable after accelerated thermal ageing

High voltage direct current (HVDC) cables develop rapidly due to the increased demand for renewable sources and long distance power transmission, recently. Cross-linked polyethylene (XLPE) insulated direct current (DC) cable is one of main HVDC cables. However, XLPE insulation materials incline to ageing and degradation in the long-term high temperature environment, which leads to the reduction of the electric, physicochemical and mechanical properties. In this paper, the effects of a single thermal ageing on the DC electrical conductivity of HVDC cable XLPE insulation materials were investigated in 130, 140 and 150
^o
C by accelerated ageing experiments in nitrogen atmosphere to simulate the pressure environment of cable insulation. The change trend and mechanism of DC electrical conductivity at different ageing temperature and time were analyzed through Fourier transform infrared (FTIR) spectra. The experiments and analysis results indicated that the electrical conductivity of XLPE insulation after thermal ageing firstly reduced and were below that of unaged XLPE, and then gradually increased during the whole ageing temperature and time range by the combined effect of thermal decomposition, post-crosslinking and the diffusion of the low molecular weight substance. The higher the thermal ageing temperature was, the greater the changes of the electrical conductivity of XLPE insulation were. Under the same ageing time of 700 h, the DC electrical conductivity of XLPE insulation at different ageing temperature followed well the Arrhenius equation. Based on data processing and analyzing, the electrical conductivity activation energies of XLPE before and after thermal ageing were obtained and it was found that the activation energies for aged XLPE insulation were superior to unaged XLPE. The thermal ageing increased the electrical conductivity activation energy. In the FTIR spectra, it can be seen that the intensity of carbonyl absorption peak at about 1722 cm
^-1
increased after thermal ageing. The carbonyl index has been calculated by the rate of peak near 1722 and 2020 cm
^-1
to evaluate the ageing degree of XLPE insulation materials. It was found that the carbonyl index of unaged XLPE was the minimum and increased as the ageing time under different thermal ageing temperature. At high thermal ageing temperature, the carbonyl index increased rapidly. The high temperature contributed to the thermal decomposition of XLPE insulation materials, which caused oxygen groups increasing in molecular chain and the increasing rate of the carbonyl was accelerated.