A Fault Current Limiter (FCL) is a device which limits the prospective fault current when a fault occurs (e.g. in a power transmission network). The term is generally applied to superconducting devices, whereas non-superconducting devices (such as simple inductors or variable resistors) are typically termed Fault Current Controllers. (For example, the ground fault circuit interrupter is commonly used in residential installations.)
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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.
FCLs are under active development. In 2007, there were at least six national and international projects using magnesium diboride wire or YBCO tape, and two using BSCCO-2212 rods. Countries active in FCL development are Germany, the UK, the USA, Korea and China. In 2007, the US Department of Energy spent $29m on three FCL development projects.
Low temperature superconductors cannot be used for commercial FCLs as the AC losses at liquid helium temperatures mean that the cryogenic cooling cost makes the whole device uneconomic.
First applications for FCLs are likely to be in electric-drive ships: naval vessels, submarines and cruise ships. Many more FCLs will eventually be used to help control land-based electricity distribution and transmission systems.