Space Charge Measurements in XLPE Cable with a Temperature Gradient

The excellent electrical properties of cross-linked polyethylene (XLPE) in combination with its good physical properties have attracted many manufacturers worldwide to explore its application for high voltage direct current (HVDC) underground cables. However, its ease of trapping electrical charge can give rise to space charge within the bulk of the material, resulting in localised electric stress enhancement which may lead to premature failure of the cable well below the anticipated and designed values. The issue has attracted many researchers over the world to investigate the mechanisms of space charge accumulation and some understandings have been gained from the research on thin film and plaque samples. Less attention has been paid to space charge dynamics in the actual cables presumably due to fewer cable space charge measurement systems available. In this paper the space charge behaviour in a commercial XLPE power cable was experimentally studied using the pulsed electroacoustic (PEA) technique. To reflect the normal operating conditions of dc cables, space charge measurements were carried out at elevated temperature with a temperature gradient of 13 oC across its insulation. The results were compared with those obtained at room temperature. Space charge measurements at room temperature on as-received XLPE cable and degassed cable revealed that the former showed a very slow charge accumulation but with a high stability, whilst the degassed sample showed little space charge formation in the bulk of the insulation. These results suggest that the cross-linking by-products play a key role in the space charge formation in XLPE insulation. Space charge formation in the bulk of the insulation at elevated temperature was quite different. The amount of charge over the same stressing period was less than that at room temperature. Charge build up rate during the stressing and decay rate after the removal of the applied voltage were enhanced. Further investigation into the effects of voltage polarity reversal on space charge at room temperature and elevated temperature indicated that the polarity change in dc transmission could pose a major thread to the cable insulation in the presence of space charge. The electric field distortion caused by the space charge at the moment of polarity switching was significant which could exceed the breakdown strength of the insulation, leading to premature failure