Investigation of space charge behavior of HVDC XLPE cables using PEA method

Space charge accumulation is a key factor concerning the safe operation of HVDC cables. In order to investigate space charge behavior of HVDC cross-linked polyethylene (XLPE) cables, degassed specimens as well as fresh specimens obtained from HVDC XLPE cables before and after type tests were tested by PEA measuring system. The measurement was performed under DC electric fields of 10kV/mm, 20kV/mm, 30kV/mm and 50kV/mm at room temperature with both stressing and short-circuit time of 1800s. A large quantity of hetero-charges were observed in the vicinity of the electrodes in fresh specimens while only a small quantity of hetero-charges appeared under 50kV/mm in degassed specimens. The field enhancement factors (defined as FEF) of fresh specimens were also higher than that of degassed specimens. These phenomena lead to discussions on the effect of degassing. Meanwhile, assessment is also made of the aging extent caused by type tests. It is demonstrated by data analysis that hetero-charge density is closely associated with crosslinking byproducts. Degassing can effectively reduce byproduct levels and thus reduce space charge accumulation. For specimens obtained from the central section of the cables, changes in space charge distribution before and after type tests are not distinct. Therefore, the aging extent of the central section of the cable after type tests is rather minor.