Review on the Impact of Fault Current Limiters on the Safety and Stability of Power Grids and Their Application Research

1 Introduction to Fault Current Limiter (FCL) Technology

Traditional passive fault current limitation methods—such as using high-impedance transformers, fixed reactors, or split-busbar operation—suffer from inherent drawbacks, including disruption of grid structure, increased steady-state system impedance, and reduced system security and stability. These approaches are becoming increasingly unsuitable for today's complex and large-scale power grids.

In contrast, active fault current limitation technologies, represented by Fault Current Limiters (FCLs), exhibit low impedance during normal grid operation. When a fault occurs, the FCL rapidly transitions to a high-impedance state, effectively limiting the fault current to a lower level, thus enabling dynamic control of fault currents. FCLs have evolved from the traditional concept of series reactor-based current limitation by integrating advanced technologies such as power electronics, superconductivity, and magnetic circuit control.

The fundamental principle of an FCL can be simplified into the model shown in Figure 1: during normal system operation, switch K is closed, and no current-limiting impedance is introduced by the FCL. Only when a fault occurs does K rapidly open, inserting the reactor to limit the fault current.

Most FCLs are based on this fundamental model or its extended variants. The primary differences among various FCLs lie in the nature of the current-limiting impedance, the implementation of switch K, and the associated control strategies.

2 FCL Implementation Schemes and Application Status

2.1 Superconducting Fault Current Limiters (SFCLs)

SFCLs can be classified as quench-type or non-quench-type based on whether they utilize the superconductor's transition from superconducting to normal state (S/N transition) for current limiting. Structurally, they are further categorized as resistive, bridge-type, magnetically shielded, transformer-type, or saturated-core types. Quench-type SFCLs rely on the S/N transition (triggered when temperature, magnetic field, or current exceeds critical values), where the superconductor shifts from zero resistance to high resistance, thereby limiting fault current.

Non-quench-type SFCLs combine superconducting coils with other components (e.g., power electronics or magnetic elements) and control operational modes to limit short-circuit currents. Practical application of SFCLs faces common superconducting challenges such as cost and cooling efficiency. Additionally, quench-type SFCLs have long recovery times, potentially conflicting with system reclosing, while non-quench-type SFCLs' impedance changes may affect relay protection coordination, requiring re-setting.

2.2 Magnetic Element Current Limiters

These are divided into flux-cancellation and magnetic saturation switch types. In flux-cancellation type, two windings with opposing polarity are wound on the same core. Under normal conditions, equal and opposite fluxes cancel each other, resulting in low leakage impedance.

During a fault, one winding is bypassed, disrupting the flux balance and presenting high impedance. The magnetic saturation switch type operates by biasing the current-limiting winding into saturation (via DC bias, etc.) under normal conditions, yielding low impedance. During a fault, the fault current drives the core out of saturation, creating high impedance for current limiting. Due to complex control requirements, magnetic element limiters see limited application.

2.3 PTC Resistor Current Limiters

Positive Temperature Coefficient (PTC) resistors are nonlinear; they exhibit low resistance and minimal heating under normal conditions. During a short circuit, their temperature rapidly increases, raising resistance by 8–10 orders of magnitude within milliseconds. FCLs based on PTC resistors have found commercial use in low-voltage applications.

However, drawbacks include: high overvoltages generated during inductive current limiting (requiring parallel overvoltage protection); mechanical stress due to resistor expansion during operation; limited voltage/current ratings (hundreds of volts, a few amps), necessitating series-parallel connections and restricting high-voltage use; and long recovery times (several minutes) with short service life, hindering large-scale deployment.

2.4 Solid-State Current Limiters (SSCLs)

SSCLs are a new type of short-circuit limiter based on power electronics, typically comprising conventional reactors, power electronic devices, and controllers. They offer various topologies, fast response, high operational endurance, and simple control. By controlling the state of power electronic devices, the equivalent impedance of the SSCL is altered to limit fault current. Considered a novel FACTS device, SSCLs are gaining increasing attention. However, during faults, the power electronic devices must carry the full fault current, demanding high device performance and capacity. Coordination between multiple SSCLs or with other FACTS control systems remains a critical challenge.

2.5 Economical Current Limiters

These offer mature technology, high reliability, low cost, and automatic switching without external control. They are mainly classified into arc-current transfer and series-resonant types. The arc-current transfer type consists of a vacuum switch in parallel with a current-limiting resistor. Under normal operation, load current flows through the switch. Upon a short circuit, the switch opens, forcing current to transfer to the resistor for current limiting.

Issues include: transfer current affected by vacuum arc voltage and stray inductance; transfer time dependent on switch speed; and difficulty in current transfer at low arc voltages, requiring auxiliary devices to boost arc voltage and force current zero-crossing. Series-resonant FCLs use saturated reactors or surge arresters as switches. Under normal conditions, the capacitor and inductor are in series resonance with low impedance. During a fault, high current saturates the reactor or activates the arrester, detuning the resonance and inserting the reactor into the line for current limiting. Electromagnetic repulsion fast switches can also rapidly bypass the capacitor.

2.6 Current Status of FCL Engineering Applications

For practical value, FCLs must not only rapidly insert impedance during faults but also feature automatic reset, multiple consecutive operations, low harmonic generation, and acceptable investment and operating costs. Currently, limited by technical challenges and cost-effectiveness, despite various experimental prototypes developed worldwide, actual grid applications remain scarce, mostly limited to low-voltage, small-capacity pilot projects.

The field started earlier abroad, with notable progress in solid-state and superconducting FCL commercialization. In 1993, a 6.6 MW solid-state breaker using anti-parallel GTOs was installed on a 4.6 kV feeder at the Army Power Center in New Jersey, USA, capable of clearing faults within 300 μs. In 1995, a 13.8 kV/675 A solid-state FCL by EPRI and Westinghouse was commissioned at a PSE&G substation. For superconducting FCLs, a hybrid AC/DC FCL was developed by ACEC-Transport and GEC-Alsthom in 1998, achieving commercialization. In 1999, a 15 kV/1200 A SFCL jointly developed by General Atomics and others was deployed at a Southern California Edison (SCE) substation.

Domestic FCL research started later but progressed rapidly. In 2007, China's 35 kV superconducting saturated-core FCL, developed by Tianjin Electromechanical Holdings and Beijing YunDian YingNa Superconductor Cable Co., Ltd., underwent grid-connected trial operation at Puji Substation, Yunnan—then the world's highest-voltage, highest-capacity superconducting limiter in trial operation. For series-resonant FCLs, China's first 500 kV device, jointly developed by China Electric Power Research Institute, Zhongdian Puri, and East China Grid, was commissioned at the 500 kV Bingyao Station in late 2009, reducing short-circuit current to below 47 kA.

Globally, FCL applications are still limited to individual projects but are gaining increasing attention. Significant potential remains in research on increasing capacity, voltage withstand, material improvements, heat dissipation, cost control, and topology optimization.

3 Impact of FCL Integration on Power System Security and Stability

The rapid impedance insertion of FCLs during faults, while effectively limiting current, alters network parameters, affecting transient stability, voltage stability, relay protection settings, and reclosing. Poor control may lead to negative effects. Coordinated control and optimal configuration are essential for multiple FCLs to achieve optimal performance.

3.1 Impact on Relay Protection and Reclosing Settings

For saturated-core SFCLs, the long recovery time means significant impedance persists post-fault, potentially requiring re-setting of automatic reclosing and relay protection. Literature suggests installing quench-type SFCLs on generator and main transformer branches; although protection re-setting is needed, the persistent high impedance during recovery can act as a braking resistor, benefiting transient stability. Various distance protection setting methods accounting for SFCLs have been proposed. Solid-state FCLs can use thyristor trigger signals, bypass breaker contacts, FCL switch positions, and GAP circuits to switch zero-sequence current protection settings, addressing sensitivity issues after FCL insertion.

3.2 Impact on Transient Power-Angle Stability

While FCLs generally operate with low impedance normally and high impedance during faults, their specific operation and structure lead to varying impacts on transient power-angle stability. Solid-state and superconducting FCLs, by inserting high impedance during faults, can enhance generator electromagnetic power output and improve transient stability.

Resistive-type FCLs improve stability more than inductive types by providing damping resistance that consumes more generator power. However, improper resistance values may cause reverse power flow to the generator, worsening power deficits. Analysis shows that for faults away from the generator, inductive SFCLs become more beneficial as total transfer reactance decreases. Resistive SFCLs also show similar characteristics beyond a threshold resistance.

The impact depends on fault location and type; FCLs affect power-angle stability only when faults occur on their installed lines. For asymmetrical faults at the line start, FCL inductance benefits stability, increasing with inductance value. At the line end, if the fault is cleared quickly, FCL inductance may hinder stability, but the negative impact decreases with higher inductance for phase-to-phase and two-phase-to-ground faults. For single-phase or phase-to-phase faults near the line end, slightly extending fault clearing time makes small FCL inductance beneficial, significantly reducing swing curve amplitude compared to fast clearing.

3.3 Impact on Transient Voltage Stability

Short-circuit faults cause voltage dips, affecting equipment operation and causing economic losses. PSCAD-based analysis shows that larger FCL inductance improves voltage dip suppression within a certain range. The inherent ability of FCLs to improve fault voltage varies with network structure. On radial feeders, FCL reactance >0.5 pu can maintain voltage above 0.8 pu during faults. Local generation or reactive support near the fault bus reduces dependency on FCLs.

3.4 Coordination with Traditional Limiting Measures

Coordinating FCLs with traditional measures (e.g., reactors, high-impedance transformers) is key to practical application. An automatic optimization method using 0–1 variables for measure deployment and integer variables for capacity forms a mixed-integer programming problem, solvable by branch-and-bound methods, to guide coordinated configuration.

3.5 Optimization of Configuration

With multiple FCLs, optimizing location, number, and parameters for cost-effective performance is a research hotspot. For small grids, enumeration or methods based on power change/loss rate suffice. For large grids with multiple nodes exceeding short-circuit limits, enumeration becomes computationally intensive and inadequate for multi-objective problems (impedance, number, location).

Weighted multi-objective optimization using genetic or particle swarm algorithms is common, but results heavily depend on weight selection. Sensitivity-based methods, calculating short-circuit current changes relative to branch impedance, avoid weight dependence and help determine optimal FCL placement, number, and impedance. Since the primary goal is current limiting, optimization can focus on limiting effectiveness, ensuring selected FCL locations affect all nodes with insufficient short-circuit margin. Cost and operational losses are also critical factors in real-world optimization.

4 Development and Application Trends of FCLs

4.1 FCL Technology Research Trends

To leverage advantages and mitigate weaknesses, new research directions are emerging. Combining superconducting FCLs with energy storage is a hot topic—absorbing energy during faults and supplying it to improve power quality during normal operation, achieving dual benefits. The key lies in power conditioning system design.

To address high capacity demands, cost, and harmonics in solid-state limiters, improved topologies like transformer-coupled three-phase bridge SSCLs with bypass inductors have been proposed. Conventional FCLs lack dynamic adjustability and steady-state compensation.

A multi-functional FCL with dynamic series compensation has been proposed: normal operation uses capacitor bank switching for stepwise line compensation; during faults, GTOs or IGCTs control the limiting degree via a series inductor, enabling multi-purpose use. Series compensation must be chosen carefully to avoid sub-synchronous oscillations.

4.2 FCL Application Trends

FCLs not only limit short-circuit currents but, under suitable conditions, can enhance power-angle and voltage stability, expanding their application scope. Emerging trends include improving DC receiving-end transmission capacity, reducing commutation failure risk, enhancing power quality, and supporting large-scale renewable integration.

In multi-terminal DC systems, FCLs can limit current without affecting normal operation. For DC receiving-end grids, FCLs installed on fault propagation paths can isolate regions, block fault propagation, shorten commutation failure duration, accelerate DC power recovery, and mitigate power imbalances and power flow transfers from simultaneous multi-infeed DC failures, enhancing overall transient stability. For large asynchronous motors, integrating SFCLs in the stator circuit enables soft starting and suppresses fault current contribution, reducing voltage dips and improving transient voltage stability.

For large-scale wind integration, FCLs at wind farm connection points can improve fault ride-through capability and reduce disconnection risks. Resistive FCLs require less impedance than inductive types for stability under the same fault duration, but inductive types offer better improvement near critical stability.

As FCL technology matures, these fast-responding, multi-functional devices—limiting faults, enhancing stability, and isolating faults—will find broader applications.

5 Conclusion

FCLs effectively limit short-circuit currents but may impact power-angle/voltage stability, relay protection, and reclosing settings. Optimized configuration and coordinated control of multiple FCLs or with FACTS devices promise significant benefits. Future FCLs will extend beyond current limiting to enhancing DC transmission, reducing commutation failures, improving power quality, and supporting renewable integration.

However, technical and economic barriers delay large-scale application of high-voltage, high-capacity FCLs. Solid-state limiters, limited by device capacity and voltage ratings, are currently restricted to distribution networks. Advances in high-power self-commutating devices may overcome these bottlenecks and reduce costs.

Superconducting FCLs offer fast response and self-triggering but face high cooling costs, heat dissipation challenges, and long quench recovery times. Considering near-term feasibility and economics, economical FCLs based on conventional equipment are the preferred solution. Solid-state limiters, with lower technical barriers and maturity, represent the mainstream future direction.