Development of coaxial integrated return conductor DC extruded insulation cable and factory joint

In this paper development of 120 kV coaxial integrated return conductor DC submarine cable and factory joint (FJ) is described. For coaxial type submarine cable, it is necessary to design a new construction and manufacturing process for the FJ. The FJ of submarine cable requires sufficient mechanical and electrical performance. The FJ for single core type DC extruded cable has already been developed for DC 250 kV and 500 kV. These manufacturing processes are able to be applied to the jointing of the main conductor and the formation of the main insulation. For the jointing of the return conductor, the lap weld method was applied, and for the formation of the return insulation, the polyethylene (PE) tape wrapping mold method was applied. 120 kV coaxial type DC submarine cable with extruded insulation was then manufactured. After manufacturing, the cable and FJ were subjected to the following tests successively: (1) tensile bending test; (2) external water pressure withstand test; and (3) electrical withstand test. Test conditions of the tensile bending test and external water test were determined on the assumption that maximum water depth would be 350 m. Finally, it was verified that the cable and FJ have sufficient mechanical and electrical performance for submarine cable lines.