Fault Current Limiter - Types

Types

Superconducting Fault Current Limiters are described as being in one of two major categories: resistive or inductive.

In a resistive FCL, the current passes through the superconductor and when a high fault current begins, the superconductor quenches: it becomes a normal conductor and the resistance rises sharply and quickly. This extra resistance in the system reduces the fault current from what it would otherwise be (the prospective fault current). A resistive FCL can be either DC or AC. If it is AC, then there will be a steady power dissipation from AC losses (superconducting hysteresis losses) which must be removed by the cryogenic system. An AC FCL is usually made from wire wound non-inductively; otherwise the inductance of the device would create an extra constant power loss on the system.

Inductive FCLs come in many designs; the simplest is a transformer with a closed superconducting ring as the secondary. In un-faulted operation, there is no resistance in the secondary and so the inductance of the device is low. A fault current quenches the superconductor, the secondary becomes resistive and the inductance of the whole device rises. The advantage of this design is that there is no heat ingress through current leads into the superconductor, and so the cryogenic power load may be lower. However, the large amount of iron required means that inductive FCLs are much bigger and heavier than resistive FCLs.

The quench process in the superconductor is different in detail between superconductors. Some superconductors quench directly in response to a high current density. High temperature superconductors quench in FCLs because a small amount of non-superconducting current heats the material and raises it above the critical transition temperature.