Electric field analysis on the insulation design of the stop joint box for DC HTS power cable

•The difference between AC and DC electric field distribution was confirmed.•Three kinds of SJB models were considered and electric field have been conducted.•An initial SJB model was designed based on the SJB for AC oil-filled cable.•In order to reduce the electric field, two kinds of improved SJB models were considered.•One step structure of epoxy spacer is more appropriate than multi-steps structure. DC HTS power cable is a promising electric power transmission line for the future of smart grid, and it has been competitively investigated at many research institutes all over the world. For the commercialization of DC HTS cable, higher power transmission capacity and longer length for long distance transmission line should be prepared. In order to meet the needs of long distance DC HTS cable, a joint box should be developed for the connection of cable components. As for AC HTS cable, a number of patents of nominal joint box have been already reported. However, any conceptual designs of the joint box for DC HTS cable have not been suggested yet. One of the reasons is that the cryogenic high voltage insulation design, especially in DC environment is not fully investigated yet. Conventional normal joint box for AC HTS cable could not be directly applied to DC HTS cable because different electric field distributions compared to AC electric field which requires totally different electrical insulation design concepts. In this paper, in order to establish the basic insulation design of the stop joint box (SJB) for DC HTS cable, three kinds of SJB models were designed and electric field analyses have been conducted both considering AC and DC environment. And the critical factors affecting the DC insulation design of the stop joint box were analyzed. From the simulation results, it was observed that the electric field distribution was totally different both in AC and DC operating conditions. And it was possible to find the weakest regions in the insulation design of the SJB. Consequently, based on the DC electric field analysis, the insulation design criteria and the desirable configurations were suggested for the insulation design of the stop joint box for DC HTS cable.