Effects of degassing duration on space charge and DC conductivity in HVDC XLPE cable insulation

High voltage direct current (HVDC) extruded cables are developing rapidly in the last two decades and cross-linked polyethylene (XLPE) has been the preferred insulation material worldwide. The central issues in design and operation of HVDC XLPE cables are the space charge and DC conductivity characteristics, which rely on crosslinking byproducts within the insulation. Degassing process during HVDC extruded cable manufacture can efficiently reduce the crosslinking byproducts content. In this paper, the effects of degassing duration on space charge behavior and DC conductivity property in cable insulation are investigated. The space charge accumulation and DC conductivity are measured with 30 and 50kV/mm electrical stress at room temperature. It is found that hetero-charges accumulate near both electrodes in undegassed specimens, while in the preliminary stage of degassing process the amount of accumulated charges decreases and increases in the later stage. DC conductivity changes in a similar way as the space charge results. For a certain degassing time, the inner insulation layer has more charge accumulation and higher conductivity compared to middle and outer insulation layers. It can be concluded that space charge characteristics and conductivity property are first improved by degassing process and then degraded somewhat with longer degassing time; and the degassing effect of outer insulation layer is better than that of inner insulation layers. More studies should be done to find out the reasons for space charge and DC conductivity changes with degassing duration, as well as the mechanisms of the migration of crosslinking byproducts in thick insulation layer in cables.