The impacts of degassing on space charge characteristics and DC conductivity in semicon-bonded XLPE for HVDC cable applications

High voltage direct current (HVDC) cables are one of the key components for the HVDC power transmission system. HVDC cables for higher voltage level, larger power capacity and higher reliability are still desired for the power industry as part of a global power strategy. One of the major concerns related to HVDC applications is the presence of space charge within the dielectrics, which distorts the electric field distribution and contributes to accelerated ageing and consequent failure of the cable insulation. The byproducts generated from the crosslinking process during the manufacture of XLPE insulation for HVDC cable are one of the major sources of hetero-charge formation in dielectrics. Therefore, degassing is widely applied in cable manufacture for the removal of such crosslinking byproducts. In this paper, composite XLPE samples were produced that include two layers of semiconductive polymer (semicon), which are thermally bonded on the top and bottom of the XLPE insulation, to mimic a real cable manufacturing process. The samples were degassed under vacuum and elevated temperature for 1, 3 and 6 days and their space charge characteristics and conductivities were then measured at an applied field of 40 kV/mm. The influence of the degassing period and the semicon material formulation on space charge formation and distribution in the system are discussed and the findings are compared with undegassed samples.