Effects of Revulcanization on XLPE Crystalline Morphology and AC Breakdown Performance

The extruded insulation submarine cable of large length employs a number of factory joints for connection in sea-crossing transmission projects. The vulcanization process of factory joint will inevitably involve reheating the cable insulation near the joint position, which may cause change of the crystalline morphology and dielectric properties of crosslinked polyethylene (XLPE). This phenomenon is especially obvious in cable with large insulation thickness since the required heating time is longer than others. This paper reports the influence of factory joint vulcanization process on crystalline morphology and AC breakdown performance of XLPE from cable near joint. XLPE of cable bulk and cable near joint were divided into inner, middle and outer parts due to their distinct thermal history in practical situation. Then the crosslinking degree, crystalline morphology and breakdown field strength at power frequency in each insulation position were measured. The results show that there is a revulcanization in cable near joint, which leads to the slight increase on the crosslinking degree, while the corresponding crystallinity reduces and the spacing of crystal plane as well as the crystal integrality increases compared with cable bulk. Moreover, the divergence of crosslinking degree and crystalline morphology between cable bulk and cable near joint exhibit more evidently in inner insulation, and AC breakdown field strength of cable near joint at power frequency is slightly lower.