Series Resonant Fault Current Limiter Based on Conventional Components: An Economical and Reliable Short-Circuit Current Solution
- Introduction: Research Background and Core Objectives
- Severity of the Short-Circuit Current Problem
With the continuous expansion of power grid scale and the constant growth of its capacity, the system short-circuit current level has risen sharply, approaching or even exceeding the withstand limits of existing equipment.
• Data Support: Monitoring indicates that the prospective short-circuit current at some 500kV, 220kV, and even 10kV substations domestically has exceeded 100 kA; the maximum periodic component of the short-circuit current at major power sources reaches as high as 300 kA.
• Serious Hazards: Extremely high short-circuit currents result in a lack of suitable high-voltage circuit breaker models, cause damage to electrical equipment due to exceeding thermal and electrodynamic force limits, and can also lead to safety issues such as electromagnetic interference in communication systems, ground potential rise, and step voltage. This has become a key technical bottleneck restricting the safe and economical development of the power grid. - Limitations of Existing FCL Technologies
Current mainstream fault current limiter (FCL) technologies have inherent drawbacks, making large-scale application difficult:
• Superconducting FCL: Relies on superconducting materials, a technology that is not yet mature, offers low reliability, involves high operation and maintenance costs, and is economically unfavorable, preventing its engineering application in the short to medium term.
• Power Electronic FCL: Limited by the voltage withstand and current-carrying capacity of power semiconductor devices, faces challenges in series/parallel voltage and current sharing control, features a complex system structure (requiring additional current-limiting components and fast protection circuits), and is costly. - Core Objective of This Research
To address the above issues, this study aims to propose a series resonant fault current limiter solution based on conventional electrical components, which is non-superconducting and non-power electronic. Specifically, two topologies are studied: - Series Resonant FCL based on a Saturable Reactor
- Series Resonant FCL based on a ZnO Arrester
This research will use Electromagnetic Transients Program (EMTP) simulation to deeply analyze their transient current-limiting characteristics, perform a comparison, and ultimately verify their significant advantages in technical feasibility, economy, and operational reliability.
II. Series Resonant FCL Based on Saturable Reactor
- Circuit Topology and Working Principle
• Topology Structure: The core consists of a saturable reactor LB, a capacitor C, and a series reactor L. LB is connected in parallel with C, and this combination is then connected in series with L into the system.
• Working Principle:
o Normal Operation: The line current is small. LB operates in the unsaturated region (its equivalent inductance LB1 is very large). Its parallel combination with C behaves inductively. Together with the series reactor L, they satisfy the power frequency series resonance condition (ωL - 1/ωC ≈ 0). The device presents very low impedance, resulting in minimal system losses.
o Fault State: A surge in short-circuit current rapidly saturates LB (its equivalent inductance drops sharply to LB2). Its parallel branch effectively short-circuits capacitor C, thus breaking the resonant condition. At this point, the series reactor L and the saturated reactor LB2 are both inserted into the system, effectively limiting the short-circuit current.
o Fault Clearance: After the fault is cleared, the current decreases. LB automatically exits saturation, the capacitor is re-engaged, and the circuit returns to the resonant state, achieving self-triggered switching without an external power source.
• Parameter Selection Principles:
o ω²LB1C >> 1 (Ensures the parallel branch behaves inductively during normal operation)
o ωL - 1/ωC ≈ 0 (Satisfies the resonance condition for normal operation)
o ω²LB2C << 1 (Ensures the parallel branch behaves capacitively during a fault, effectively shorting the capacitor) - Current-Limiting Characteristic Simulation Analysis (EMTP)
Simulation was conducted under a single-phase-to-ground short-circuit fault condition in a 220kV system (prospective short-circuit current peak: 110kA). Key conclusions are as follows:
|
Influencing Factor |
Core Conclusion |
Typical Simulation Data (Example) |
|
1. Unsaturated Inductance LB1 |
Increasing LB1 significantly reduces capacitor overvoltage but has little effect on short-circuit current; effect saturates. |
LB1=1317mH: Capacitor voltage 270kV; LB1=1321mH: Capacitor voltage 157kV (42% decrease) |
|
2. Saturated Inductance LB2 |
An optimal range exists (1-7mH). Too small gives poor limiting; too large causes severe capacitor overvoltage. |
LB2=7mH (C=507μF, L=20mH): Short-circuit current 25kA, Capacitor voltage 157kV |
|
3. C/L Parameter Coordination |
An optimal combination exists to cooperatively control short-circuit current and capacitor overvoltage. |
Optimal combination (C=406μF, L=25mH): Short-circuit current 22kA, Capacitor voltage 142kV |
|
4. Short-Circuit Inception Angle |
Transient characteristics are highly influenced by phase angle; most severe overvoltage at 0°/180°; design must consider worst case. |
0° phase: Short-circuit current 18kA, Capacitor voltage 201kV; 90° phase: Short-circuit current 22kA, Capacitor voltage 142kV |
III. Series Resonant FCL Based on ZnO Arrester
- Circuit Topology and Working Principle
• Topology Structure: The saturable reactor LB is replaced by a ZnO arrester. The remaining structure (parallel C + series L) remains unchanged.
• Working Principle: The principle is the same as the saturable reactor type. During normal operation, the ZnO exhibits high resistance, and the circuit resonates. During a fault, the rising capacitor voltage causes the ZnO to conduct (presenting low resistance), shorting the capacitor and breaking the resonance. The series reactor L limits the current. The system recovers automatically after fault clearance. The entire process utilizes the nonlinear volt-ampere characteristics of the ZnO for automatic switching. - Current-Limiting Characteristic Simulation Analysis
Simulation under the same system conditions yielded key conclusions:
|
Influencing Factor |
Core Conclusion |
Typical Simulation Data (Example) |
|
1. Arrester Residual Voltage & C/L Coordination |
Easy to limit capacitor overvoltage, but increasing L to pursue lower short-circuit current leads to excessive voltage on the series reactor. |
C=254μF, L=40mH: Short-circuit current 20kA, Reactor voltage 246kV; C=507μF, L=20mH: Short-circuit current 35kA, Reactor voltage 173kV |
|
2. Short-Circuit Inception Angle |
Transient characteristics are insensitive to short-circuit phase angle, only affecting current magnitude; maximum current at 90°. |
90° phase (C=507μF, L=20mH): Short-circuit current 35kA; 0° phase: Short-circuit current 28kA |
IV. Comprehensive Comparison of the Two FCL Schemes
|
Comparison Dimension |
FCL Based on Saturable Reactor |
FCL Based on ZnO Arrester |
|
Core Advantage |
Superior current-limiting effect; good balance between short-circuit current and component overvoltage achievable through parameter optimization. |
Easy limitation of capacitor overvoltage; transient characteristics unaffected by short-circuit phase angle; simpler design. |
|
Core Limitation |
Requires precise optimization of core hysteresis characteristics and C/L parameters; difficult control of capacitor overvoltage; significantly affected by short-circuit phase. |
Prominent overvoltage issue on the series reactor when pursuing low short-circuit current; requires strict control of L value. |
|
Key Parameter Requirement |
Optimal equivalent saturated inductance LB2 ≈ 1/3 of the capacitive reactance. |
Inductance value of the series reactor should not be too large. |
|
Applicable Scenario Preference |
Suitable for medium-low voltage levels (e.g., 110kV) in high-voltage grids, where high current-limiting performance is required. |
Suitable for scenarios sensitive to capacitor overvoltage with moderate short-circuit current limiting requirements. |
|
Common Characteristics |
1. Simple structure: Composed entirely of conventional electrical components, no complex control; |
V. Conclusion
This study proposes two innovative series resonant fault current limiter solutions based on conventional components, successfully overcoming the technical and economic bottlenecks of traditional superconducting and power electronic FCLs.
- Saturable Reactor FCL: Through meticulous optimization of the core hysteresis loop characteristics, setting the saturated inductance value (LB2) to approximately 1/3 of the capacitive reactance, and ensuring good coordination with the capacitor and series reactor parameters, it can effectively suppress capacitor overvoltage and achieve excellent transient current-limiting performance. It is particularly suitable for medium-low voltage level grids such as 110kV.
- ZnO Arrester FCL: Utilizing the nonlinear characteristics of ZnO easily limits capacitor overvoltage, and its performance is unaffected by the short-circuit phase angle. However, attention must be paid to avoiding overvoltage on the series reactor itself caused by excessive L values. It is more suitable for occasions with high requirements for capacitor safety and moderate current-limiting needs.
