Fault current limiter
Encyclopedia
A Fault Current Limiter 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.)
.
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.
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.
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
, 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.)
Types
Superconducting Fault Current Limiters are described as being in one of two major categories: resistive or inductiveInductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
.
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
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
or AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
. If it is AC, then there will be a steady power dissipation from AC losses (superconducting hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
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
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
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.
Development
FCLs are under active development. In 2007, there were at least six national and international projects using magnesium diborideMagnesium diboride
Magnesium diboride is a simple ionic binary compound that has proven to be an inexpensive and useful superconducting material.Its superconductivity was announced in the journal Nature in March 2001. Its critical temperature of is the highest amongst conventional superconductors...
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
Liquid helium
Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
temperatures mean that the cryogenic cooling
Cryogenics
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...
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
Electricity distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...
and transmission
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
systems.