Fault Current Limiters (FCLs) act as an additional high impedance to limit high fault currents to an acceptable level. In normal operation, FCLs have almost no impedance and are ‘invisible’ to the system. Un-like fuses or disconnectors, FCLs do not completely disconnect in a fault case. After the fault current dis-appears, they can return to normal operation. Due to their very short reaction time, FCLs can act and re-duce the short circuit current so the CB can act in its nominal performance range if required. Typically, FCLs are used in low and medium voltage levels.
FCL can be classified into two major types, which can be further subcategorised per technology
Components & enablers
- Saturable core FCL – windings of conventional conductors, iron core, air-core reactor.
- Solid state FCL – solid state switch consisting of a configuration of semiconductor devices e.g. SCR (Silicon Controlled Rectifier), GTO (Gate turn-off thyristor), IGBT (Insulated Gate Bipolar Transistor), IGCT (Integrated Gate-Commutated Thyristors).
Super conducting FCL (SFCLs)
- Resistive type of SFCLs – air-core superconductor transformer and PWM (Pulse-width modulation) converter.
- Inductive type of SFCLs – power semiconductors devices in parallel with a resistor or inductor.
- Bridge type of SFCLs – diodes and thysistor, DC reactor.
Advantages & field of application
By installing FCLs, system operators or commercial customers can optimise the system via the application of standard solutions with specific (low) nominal short circuit currents. Major advantages include:
- Reduction of the short-circuit current of the system, allowing the CBs to act in their nominal performance range
- Reduction of voltage sags and flicker due to the lower total source impedance
- Reduction of harmonics due to the lower total source impedance
Technology Readiness Level
HV (≤ 145 kV)
Non-superconducting FCL : TRL 9
Super conducting FCL (SFCLs): TRL 3 - TRL 7 depending on technology types and voltage level
EHV (≥ 245): TRL 3
Research & Development
Current fields of research: Superconducting materials, compactness, self-triggering, increasing maximum voltage level, time to return to normal operation, cost reduction.
Best practice performance
Voltage rating: up to 138 kV
Current rating: up to 6.3 kA
Limiting capability: 210 kA (RMS)
Best practice application
Designing, building and testing of an FCL prototype in Tidd substation of American Electric Power.
A three phase 138 kV, 1300 A saturable iron-core type 2G HTS FCL unit that reduces a 20 kA fault current by 43 % and instantaneously recovers under load was installed on the LV side of a 345 kV/138 kV transformer to protect the feeder.
By employing fault current limiters, the electrical installed equipment is protected.
UK, Newhaven, East Sussex
The Energy Technologies Institute worked on implementing an FCL in a main substation of UK Power Networks to supress damaging currents and provide network capability and reliability.
A novel design was demonstrated that provides a simple, reliable and low maintenance solution and is fully scalable to other voltage levels. The concept of magnetic flux alteration for the iron core saturation is used. Instant, self-triggering response to new faults and quick recovery after clearance without interruption in the network as well as being able to cope with multiple consecutive faults are all benefits of the chosen design.
Shorter connection times and reduced costs of connection with increasing shares of embedded generation. Increased efficiency and flexibility and resilience of the electric network is achieved.
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