The FCL is a unique device, with no classical equivalent, that automatically and naturally limits current when it exceeds a predetermined value. It can be compared to a permanent superconducting fuse since it automatically resets after a fault (a short-circuit for instance). The FCL is based on the intrinsic and almost instantaneous transition from a no-resistance state to a strong-resistance state in a superconducting element when the current exceeds a certain value called “critical current”.
All you have to do is insert in the electric line to be protected a superconducting element with a critical current properly chosen considering the assigned (normal) current of the line. Usually, the length is chosen according to thermal considerations. The arrangement of this length must be done in order for the inductance to be minimum: the limiter must be almost invisible at a normal use. A circuit breaker is added to the superconducting element for the fault current to be completely insulated. Once it has been insulated, the superconductor can recover its nondissipative state and can be brought into service again on the line. If the superconducting/dissipative transition is extremely fast (millionths of a second), the recovering process takes more time and can reach several minutes. Some schemes enable to by-pass this inconvenient in order to guarantee service continuity (very quick recovering process of the line after the fault, of about hundredths of a millisecond).
Today, fault currents are not limited but only broken by circuit breakers. The current situation is not satisfying and presents many constraints for electric networks. The superconducting fault current limiter is the ideal device the creators of electric networks were dreaming about. It unlocks fault currents in networks, enabling better security and better quality of the electric energy, two demands that are currently very strong.
Two superconducting current limiters are working very well today in the European network: one in the United Kingdom, the other in Germany.
Superconducting fault current limiters have a very promising future, whether it be in current networks or future networks.
Evolution of a current during a fault, with and without a current limiter.
