Two-stage cryocooling design for hybrid superconducting fault current limiter

New concept of two-stage cryogenic cooling system for full-scale hybrid superconducting fault current limiter (SFCL) is proposed and designed with an objective to significantly reduce the capital and operational costs. Even though we have developed and successfully tested a cryocooling system for 300 A level, it is not easy to scale up the same technology to 3φ/22.9 kV/2.5 kA level, mainly because of the heavy cost of GM or Stirling cycle coolers to cover the increased thermal load. It is proposed in this study to employ a large capacity of JT cooler for heat intercept at an intermediate location of current leads. The JT coolers with mixed refrigerant are very reliable and commercially available at an inexpensive price. Detailed cryogenic design of cryostat and heat intercept components is carried out in consideration of thermal characteristics of cryocoolers and electrical insulation. It may be concluded that the two-stage cooling with a JT cooler at 130 K and a GM cooler at 77 K is feasible in practice, and can significantly reduce the capital cost and power consumption.