Cooling Unit for the 500 kV Saturated Iron Core Fault Current Limiter

The high temperature superconducting dc bias winding of the single-phase 500 kV saturated iron core is designed to be cooled with liquid nitrogen down to a temperature of 72 K. To achieve this temperature level, the coolant has to be vaporized below atmospheric pressure. An open cryogenic system was designed and fabricated, which composed of a cryostat, heat insulation pipelines, a liquid nitrogen tank, a control circuit, and a vacuum pump. Since the dc bias winding is composed of two YBCO windings and one Bi-2223 coil, three pairs of current leads are assembled on the cryostat. A control system is used to maintain the liquid nitrogen level and the operation temperature by regulating the pumping rate. According to the testing results, the steady-state heat loss including the cryostat, the conduction heat loss of current lead is about 135 W, and the total heat loss including the Joule heat loss is about 187 W at 77 K.