Experimental Design and Test of 100 kV Extra High Voltage Prototype Bushing With ${\rm CF}_{4}$ as Insulation Gas for Superconducting Equipment

One of the critical matters yet to be solved for commercial applications of extra high voltage superconducting devices is the optimum design and development of high voltage cryogenic bushings which could withstand both severe insulation requirements and a steep temperature gradient due to the cryogenic environment. Neither conventional extra high voltage bushings insulated by SF₆ gas nor composite materials are directly applicable to cryogenic bushings due to an extremely low temperature environment. In order to obtain suitable dielectric performance of bushings in the cryogenic environment, we focused on an alternative insulation gas instead of SF₆ such as CF₄, which shows excellent dielectric performance under extremely low temperatures, and also on the optimum design of cryogenic bushings, which have a longer creepage distance compared to conventional bushings. In this paper, design factors of cryogenic bushings were discussed, and test results of 60 and 100 kV extra high voltage prototype bushings were discussed in detail. Consequently, it was possible to obtain satisfactory results to verify the insulation level of newly designed extra high voltage cryogenic prototype bushings for superconducting electric power applications.