Interface Characteristic of Extrusion-Molded joints of 500kV Submarine Cables

As a key accessory made from the crosslinked polyethylene (XLPE) in high-voltage (HV) insulated submarine cable, the EMJ joints represent unpredictable uncertainty in production reliability due to workmanship issues. Dielectric cracks in the size of tens of micrometers in insulation interfaces are identified as a string of voids to dominate insulation damages. The abnormal arrangements of XLPE lamellae observed by scanning electron microscope (SEM) imply that the structural micro-cracks will be formed under interface stresses. Electrical tree inception is facilitated to a faster propagation due to the poor dielectric property of the interface region, manifesting as 30% lower of tree inception voltage, which is attributed to the longer free-paths for accelerating charge carries in the cracks of interface region and will stimulate partial discharging from needle electrodes. The carbonized micro-discharging channels only arising in interface region suggest that the micro-discharging channels can be triggered by the voids in cracks and will finally develop into electrical breakdown of insulation damage. The mechanism of forming cracks in the fusion processes between the molten XLPE of cable and the molten XLPE of recovery insulation is elucidated.