Operation of an SCFCL at 65 K

The SuperConducting Fault Current Limiter (SC FCL) is a very promising application with the first commercial sales for permanent uses in electrical grids. The reduction of the cost of the SC tape is indispensable to increase the deployment of SC FCL, especially for high-voltage devices such as for HVdc supergrids. The cost reduction is obtained by three ways: the cost reduction of the tape, the increase of the electric field under limitation, and the lowering of the temperature. The use of liquid nitrogen as a coolant has many advantages: industrial low-cost coolant, good exchanges, satisfying dielectric properties, etc. This is why we have chosen to study the operation with 65 K subcooled liquid nitrogen. Measurements show that recovery is then very quick and the bubbles during limitation rapidly collapse, at 65 K (the critical current is multiplied by 2 compared to 77 K). However, the superconductor is more sensitive to hot spots for low prospective current faults. We have studied this hot spot behavior from simulation (multiblock one-dimensional model) and experimental points of view including a high-speed camera. We propose a preliminary design of a RECO conductor able to withstand 150 V/m (40 ms) and safe from a hot spot point of view. The favorable influence of the increase of the normal zone propagation velocity is tackled.