Short-Circuit Current Limitation in Power Systems and the Application of Fault Current Limiters (FCL)

0 Introduction​
With the development of power systems and increasing load demands, the integration of large-capacity generating units and substation equipment—particularly the emergence of large power plants in load centers and the interconnection of large power systems—has inevitably led to a continuous rise in short-circuit current levels. Without effective limitation measures, this trend would not only significantly increase the equipment investment for new substations but also severely impact communication lines and pipelines of existing substation facilities, potentially requiring substantial funds for renovation and upgrading.

In the early stages of system development, when system capacity is small and short-circuit current levels are low, increasing short-circuit currents can usually be addressed by replacing switching devices—other substation equipment often has sufficient margin at this stage. However, when the power system capacity is large, short-circuit levels are high, and short-circuit currents continue to rise due to system interconnection or further capacity expansion, simply replacing circuit breakers is no longer sufficient. Existing substations may require not only circuit breaker replacements but also enhancements or replacements of main transformers, disconnectors, instrument transformers, busbars, insulators, structures, foundations, and grounding systems. Additionally, communication lines may need shielding or even conversion to underground communication cables.

Due to various factors, new large-capacity generating units and power plants continue to be integrated into the 220kV grid, leading to an excessively rapid increase in short-circuit current levels. The interrupting capacity and dynamic stability performance of numerous 220kV circuit breakers—and even entire substations—can no longer match the rising short-circuit levels, creating serious technical and economic challenges. Research on short-circuit current limitation is therefore urgently needed.

​1 Traditional Current Limiting Measures and Their Limitations​
Short-circuit current limitation can be addressed from the perspectives of system structure, operation, and equipment. Traditional measures include the following categories, but each has significant limitations:

• ​a. Grid Structure Adjustment​
  Includes developing higher-voltage grids, splitting low-voltage grids/busbars, and grid separation.
• Developing higher-voltage grids: Requires large investments and involves environmental concerns.
• Low-voltage grid splitting/separation: Simple to implement with significant current-limiting effects but reduces system safety margins and limits operational flexibility, making it suitable only for necessary scenarios.
• ​b. DC Interconnection Technology​
  DC interconnection can significantly reduce short-circuit currents, but the investment in converter stations at both ends is extremely high. For short interconnections with low power exchange, this solution is economically unfeasible.
• ​c. High-Impedance Transformers​
  Using high-impedance transformers to limit short-circuit currents on the low-voltage side is a commonly adopted measure. However, these transformers exhibit higher losses during steady-state operation, affecting system economy.
• ​d. Series Reactors​
  Series reactors, with mature manufacturing technology and clear current-limiting effects, are already used in power plant auxiliary systems and 10–35kV substations. However, their application in ultra-high-voltage systems increases network losses and reduces system stability, limiting their suitability.
• ​e. Equipment Capacity Expansion and Retrofitting​
  Replacing circuit breakers and retrofitting existing substations to handle higher short-circuit currents directly address the issue but involve high investment and complex construction, resulting in poor economic efficiency and timeliness.

Given the significant limitations of traditional measures, developing new current-limiting devices adapted to modern power systems has become imperative. The Fault Current Limiter (FCL) has emerged as a solution and is also an important component of Flexible AC Transmission Systems (FACTS).

​2 Application of Fault Current Limiters (FCL) in Power Systems​

​2.1 Model and Basic Principles of FCL​
The basic principle of FCL is derived from series reactor current-limiting technology, improved with power electronics to overcome the drawbacks of traditional series reactors (e.g., high steady-state losses and impacts on system stability). Its core model can be abstracted as: "No reactance under normal operation; rapid insertion of reactance during faults to limit current."

• Normal operation: Switching device closed, FCL equivalent impedance near zero, no impact on the system.
• Fault condition: Switch rapidly opens, inserting the current-limiting reactor to suppress short-circuit current.

The core components of FCL include four key elements:

1. Fast fault current detection element: Monitors system current in real time and quickly identifies short-circuit faults.
2. Fast switching device: Acts rapidly during faults to switch between "no reactance" and "reactance" states.
3. Current-limiting reactor: Core current-limiting component, suppressing short-circuit current through impedance.
4. Overvoltage protection element: Prevents overvoltage during fault switching, protecting system equipment.

​2.2 Functions and Design Requirements of FCL​

​2.2.1 Core Functions of FCL​
FCL provides a new approach to fault current limitation in power systems and is a critical component of modern power systems. Its advantages include:

• Reducing circuit breaker burden: Higher voltage levels correspond to larger, harder-to-interrupt fault currents. FCL directly reduces the interrupting current of circuit breakers, extending equipment lifespan.
• Improving system stability: Rapidly limiting short-circuit currents reduces line voltage drops and generator out-of-step probabilities, enhancing power angle, voltage, and frequency stability.
• Increasing equipment and line utilization: If FCL acts before the short-circuit current peaks, it reduces requirements for thermal and dynamic stability limits, thereby increasing the actual transmission capacity of lines.
• Optimizing voltage quality: Rapid current limitation before fault clearance shortens voltage sag duration on non-faulted lines, ensuring grid voltage stability.
• Reducing interference with surrounding facilities: Limiting short-circuit currents in high-voltage grids reduces electromagnetic interference with nearby communication lines and railway signaling systems.

​2.2.2 Design Requirements for FCL​
To adapt to power system operating characteristics, FCL must meet the following design standards:

• No impact on the system during normal operation (voltage drop near zero).
• Fast response during faults (within 1–2 ms), limiting both peak and steady-state short-circuit currents without side effects such as overvoltage.
• Automatic reset after fault clearance without manual intervention.
• No interference with the normal operation logic of protective relays.
• Reasonable cost and high cost-effectiveness, meeting utility engineering application needs.

​2.3 Comparison of Various FCL Implementation Schemes​

​2.3.1 Scheme Comparison​

• ​Conclusion: New material-based (especially superconducting) and power electronics-based FCLs are currently the optimal solutions. The former is simple and reliable but limited by material technology; the latter offers strong controllability, and with declining power electronics costs, it has become engineering feasible, making it the most promising R&D direction.

​2.5 Future Research Directions for FCL​
Future research on FCL should focus on "performance optimization, functional integration, and engineering adaptation." Key directions include:

1. Continuously adjustable impedance converters: Moving beyond the current "two-state impedance (zero or infinite)" limitation to develop responsive, continuously adjustable impedance converters that dynamically match higher impedance with larger fault currents. These should also incorporate power factor compensation and overvoltage absorption, combined with control theories (e.g., negative feedback, PID control) to enhance system automation.
2. Integration with FACTS controllers: Developing comprehensive control devices that combine FCL with other FACTS components (e.g., SVG, SVC) to improve overall cost-effectiveness and advance controllable AC transmission and distribution systems.
3. Key technology breakthroughs:
• Impact mechanisms of FCL on power system stability.
• Coordination logic between FCL and protective relays.
• Optimization of ultra-fast fault signal detection systems and controllers.
• Effects of FCL on power quality (e.g., harmonics, voltage fluctuations) and mitigation measures.

​3 Conclusion​

• a. Short-circuit current limitation in power systems has become a critical issue requiring urgent resolution. As a new protection device, the Fault Current Limiter (FCL) offers an effective solution, and developing FCLs adapted to modern grids holds significant theoretical and engineering value.
• b. Power electronics-based FCLs already possess a theoretical foundation and engineering practicality. Their excellent control performance and declining costs of power electronic devices indicate broad development prospects.
• c. With the advancing development of FACTS/CusPow technologies, FCL—as a key member of the FACTS family—should not only independently address current limitation issues in transmission and distribution grids but also collaborate with other FACTS controllers to further promote the development of controllable AC transmission and distribution systems.