Installation and Application of Fault Current Limiters in Power Systems

1 Locations for Installing Fault Current Limiters (FCLs)

• At Generator Terminals：Installing an FCL at this location reduces the short-circuit current level in the grid during faults, minimizes mechanical and thermal stress on the generator, and consequently reduces losses in equipment and devices.
• At Plant Distribution Substations：Short-circuit current levels at this location are typically very high. Installing an FCL can significantly suppress fault currents.
• Across the Entire Busbar：When increasing load demand requires larger transformers, existing circuit breakers and disconnect switches may not need replacement. At higher power levels, high-capacity, low-impedance transformers can be used to maintain voltage regulation, while limiting fault current stress on the transformer. After limiting fault current on the high-voltage side of the transformer, a short-circuit on a medium-voltage busbar will cause only a minimal voltage drop on the high-voltage busbar.
• At Network Tie-Lines：Installing FCLs at network interconnection points provides significant benefits in terms of power flow control, voltage stability, supply security, system stability, and disturbance mitigation.
• At Busbar Interconnections：After connecting separate busbars with an FCL, the impact of short-circuit currents does not increase significantly. When a fault occurs on one busbar, the voltage drop across the SFCL helps maintain voltage levels on the faulty busbar, allowing it to remain in service. Connecting multiple busbars enables parallel operation of transformers, reducing system impedance, enhancing voltage regulation capability, and eliminating the need for tap-changing transformers. Excess power from one busbar can supply loads on another, improving the utilization of transformer rated capacity.
• At Current-Limiting Reactor Locations：Under normal conditions, the FCL shorts out the current-limiting reactor, avoiding unnecessary voltage drop and power losses.
• At Transformer Feeders：Installing an FCL at the transformer feeder protects downstream equipment and reduces inrush currents during switching operations.
• At Busbar Feeders：If an FCL is not installed at the transformer feeder, it should be installed at the busbar feeder. Although this may require more FCL units, it reduces losses on the busbar under both normal and fault conditions.
• At Local Generator Connection Points：FCLs are highly beneficial for connecting additional distributed generation sources (e.g., thermal power plants, wind farms) as they reduce the contribution of these sources to the total short-circuit current.
• For Closing Open Loops：In medium-voltage networks, loops are sometimes kept open due to high short-circuit currents. FCLs can be used to close these loops, improving supply reliability, voltage balance, and reducing network losses.

2 Research Directions for Fault Current Limiters

Currently, FCL applications are limited to individual projects. For large-scale deployment, the following research areas are urgently needed:

• Investigate the role of FCLs in enhancing power transmission capacity and their impact on grid stability; propose fundamental parameters that meet power system stability requirements.
• Study optimal installation locations and capacity configurations for FCLs based on typical regional grid structures, and determine key parameters that satisfy both system stability and equipment thermal/mechanical withstand capabilities.
• Research coordination and control strategies among multiple FCLs or between FCLs and existing FACTS devices.
• Investigate integration of FCL control with conventional system controls and relay protection schemes.
• Study methods to incorporate FCL control into existing grid dispatch and control systems.
• Analyze the mutual impacts between FCLs and the power system when deployed at various load locations, and develop corresponding mitigation strategies.
• Explore the role of FCLs in large interconnected power grids.

FCLs are high-voltage, high-power devices, and their reliability and cost-effectiveness are critical performance indicators. Improving reliability requires not only rational circuit topologies and mature control strategies but also simplicity in design and control. Optimizing system design to reduce size, weight, and cost remains a central goal in FCL research. Additionally, the anti-interference capability and operational stability of the control system are essential for reliable fault current limitation.

Another issue with FCLs is their single function—they remain inactive during normal operation, increasing grid investment costs. In distribution networks, various power quality compensation devices (e.g., Dynamic Voltage Restorers (DVR), Unified Power Quality Conditioners (UPQC), Advanced Static Var Generators (ASVG), Superconducting Magnetic Energy Storage (SMES)) are often installed to improve power quality. If a device could be designed to provide multiple compensation functions under normal conditions (improving power quality) and instantly present high impedance during system faults to limit fault current, it would achieve multi-functionality. Such a device could also offer improved current-limiting principles and performance compared to existing FCLs.

3 Current Issues with Fault Current Limiters

As a novel protective device, FCLs are receiving increasing attention, and their future application in power systems appears promising. However, analyzing their potential impacts and effects is an unavoidable challenge. Major current issues include:

• The dynamic behavior of FCLs during fault transients, including impacts on synchronism stability and load stability.
• Fault control strategies of FCLs and their coordination with relay protection systems.
• Design of ultra-fast fault detection systems and controllers for FCLs.
• Impact of FCLs on power quality, particularly concerning harmonic generation.
• Optimal integrated placement of FCLs in power systems.
• Effects of FCLs on the operational status of existing equipment and components in the grid.
• Economic evaluation of FCL applications in power systems. Addressing these issues will greatly promote the development and adoption of FCL technology.

Specific Issues for Superconducting Fault Current Limiters (SFCLs):

• Stability of superconducting magnets.
• Recovery time of superconductors after a fault.
• Heat dissipation from superconductors after current limiting.