DC Conductivity Fluctuation Due to Temperature Dependence and Cross-linking Byproducts of XLPE Insulation Material in HVDC Cables

This research clarifies the significance of DC conductivity for XLPE insulation material used in extruded high voltage DC cables. The DC conductivity is a vital dielectric property, especially for cables operating at HVDC conditions. For HVDC cables, XLPE insulation with greater thickness exhibits a specific radial inhomogeneity depending on crosslinking byproducts and morphological structure. The DC conductivity of XLPE material is influenced by the morphological structure, cross-linking byproducts, and temperature fluctuation within the XLPE insulation. The removal of cross-linking byproducts is essential to improve the cable properties. In this research, XLPE insulation is degassed for 0 to 30 days respectively to measure the DC conductivity on five sliced layers namely inner, inner middle, middle, middle outer, and outer insulation at
$\boldsymbol{30^{\circ}\mathrm{C}}$
to
$\boldsymbol{90^{\circ}\mathrm{C}}$
temperature under 10 kV/mm to 50 kV/mm field stress condition. This DC conductivity is used to analyze the DC conductive response and temperature conductivity dependence of degassed and non-degassed five-layered XLPE insulation. The cross-linking byproducts in XLPE insulation are analyzed by using Fourier transform infrared spectrum (FTIR) method. It is concluded that the byproducts such as acetophenone,
$\boldsymbol{\alpha}$
-methyl styrene, and cumyl alcohol exhibit a radial distribution in the XLPE insulated regions. Moreover, the uncertain behavior of DC conductivity with or without the degassing process is mainly due to the decreasing behavior of acetophenone. Also, it is investigated that the DC conductivity is disturbed by the variation in temperature and byproducts content, mainly acetophenone within the XLPE insulation. DC conductivity influential factors and reasons to attain uniformity of DC conductivity within HVDC XLPE cable insulation has to be further studied.