Surface Breakdown Characteristics of Composite Insulating Papers in LN2 for a Stop Joint Box of DC HTS Power Cable

One of the key components for dc HTS cable for long-distance line is a stop joint box (SJB). Up to now, the SJB has not been commercialized yet due to some difficulties of dc electrical insulation matters in LN₂. Basically, the structure of SJB would be similar to the SJB of oil-filled cable because they adopt liquid insulation and sectionalized cooling separations. The insulation structure of conventional SJB based on ac electric field was no more suitable for dc; hence, there is a need to design suitable insulation structure, which can be applied for dc. In this work, in order to find a solution to reduce dc electric field concentration on the polypropylene laminated paper (PPLP) above epoxy spacer, double layer structure composed of PPLP and Kraft were suggested. To verify the effectiveness of this structure for relieving dc electric field, simulation and experiments were performed. From analytical works, the Kraft layer between PPLP and epoxy has shown noticeable electric field mitigation. Furthermore, surface breakdown tests considering PPLP/Kraft combined layers were conducted to verify the insulation properties of double layer structure. Finally, it was deduced that Kraft layer was effective to enhance surface breakdown voltage. Surface breakdown characteristics of PPLP and Kraft were relevant to the surface roughness of insulating materials.