AbstractThe large helical device (LHD) has 12 superconducting coils that are strongly coupled with each other. The rating currents of these coils are from 13 to 32 kA and total magnetically stored energy reached 1.6 GJ. These coils are excited by six d.c. power supplies and each coil current is controlled with high accuracy to make a magnetic surface required for various plasma experiments. At the same time, it is required that these power supplies discharge the stored energy quickly when coil quench is detected. This paper introduces the quench protection circuit installed in the power supplies. For this system, a 30 kA and 2 kV class d.c. current breaker using the parallel connection of a small vacuum circuit breaker (VCB) and power fuse is developed. Also, the best discharge resistor set, that dumps stored energy in the wide operation range of the LHD without changing connection, is found using numerical simulation. To confirm the reliability of quench protection and operation delay, operating tests are done using an actual quench protection system and power supply. In this test, more than 400 shots for three deference power systems are finished without any operation failure.