Influence of Crosslinking Byproducts on DC Conductivity of HVDC XLPE Cable Insulation

Extruded high voltage direct current (HVDC) cable with cross-linked polyethylene (XLPE) insulation is a key power device which has been widely used globally due to its advantages. However, there are many challenges in the development of HVDC XLPE cable. One of the issues in design and operation is the DC conductivity of XLPE material, which could be affected by crosslinking byproducts within the insulation. In this paper, the influence of crosslinking byproducts on the distribution and the evolution of DC conductivity of HVDC XLPE cable insulation is investigated. The content of crosslinking byproducts was obtained by Fourier transform infrared spectrum (FTIR) method and DC conductivity was measured with 20, 30 and 50 kV/ mm electric stress at 30 °C. It is found that the content of cumyl alcohol, a-methyl styrene and acetophenone exhibits spatial distribution in the insulation layer and varies with the degassing process. The byproduct content of inner insulation is in the highest level, and acetophenone decreases most significantly during the degassing. Affected by this, the conductivity demonstrates corresponding distribution profile in different insulation positions which is consistent with the byproducts distribution. The variation of conductivity during the degassing process is in accord with the decrease tendency of acetophenone, since it will greatly affect the ionic conduction in insulation. Besides, crosslinking byproducts are taking effect combined with some other factors, for instance crystallinity, degree of crosslinking of XLPE material, as well as cable construction. It is concluded that the DC conductivity of HVDC XLPE cable insulation is affected by multiple factors, and conductivity design and operation issues should be considered comprehensively related to the construction and manufacturing process of HVDC XLPE cable.