Collaborative Solution for Fault Current Limiter (FCL) and Single-Phase Auto-Reclosing (SPAR) in Southeast Asian EHV Power Grids

RW Energy

08/26/2025

Introduction: Research Background and Significance
With the rapid economic development in Southeast Asia, power grid scales continue to expand, and loads keep increasing. This has led to system short-circuit currents approaching or even exceeding circuit breaker interrupting capacity limits, seriously threatening the safety and stability of power grid operations. Meanwhile, EHV transmission lines serve as the backbone for regional power interconnections. Over 70% of faults are single-phase grounding faults, and approximately 80% of these are transient faults (e.g., lightning strikes, wind-blown foreign objects). Single-Phase Auto-Reclosing (SPAR) technology is a key method for quickly clearing faults, restoring power supply, and ensuring grid stability and reliability.

Fault Current Limiters (FCLs), particularly cost-effective metal-oxide arrester (MOA)-type FCLs, are effective measures for suppressing short-circuit currents and have been gradually applied in EHV grids. However, existing research has mostly focused on the impact of FCLs on system transient stability and relay protection, neglecting their potential adverse effects on SPAR success rates. This proposal aims to fill this research gap by conducting an in-depth analysis of the interaction between FCLs and SPAR, and proposing a set of collaborative control strategies suitable for Southeast Asian power grids. These strategies ensure both effective current limiting and reliable power supply.

1. Working Principle of Metal-Oxide Arrester-Type FCL
This type of FCL primarily consists of the following components, which work in coordination to achieve the core function of "low impedance during normal operation and high impedance during faults":

Component	Function Description
Reactor Lf (Lf = Lc + L)	During normal operation, it resonates in series with capacitor Cf, presenting low impedance; during faults, the current-limiting reactor L is inserted into the system.
Capacitor Cf	Participates in resonance during normal operation; during faults, it is quickly short-circuited by the MOA and exits the resonant circuit.
Metal-Oxide Arrester (MOA)	Acts immediately upon detecting a short-circuit fault, conducting to short-circuit capacitor Cf.
Bypass Switch K	Closes quickly after a fault to share current and protect the MOA from absorbing excessive energy. Its timing is critical.
Current-Limiting Reactor Lc	Primarily limits the discharge current of capacitor Cf through the triggering gap.

Workflow: During normal system operation, Lf and Cf resonate → FCL impedance is nearly zero → no impact on power flow. When a short-circuit fault occurs, the MOA quickly acts to short-circuit Cf → the current-limiting reactor L is inserted into the system to suppress the short-circuit current → the triggering gap breaks down and sends a signal to close bypass switch K → after K closes, it diverts current to protect the MOA.

2. Problem Analysis: Adverse Effects of FCL on Secondary Arc Current and SPAR
The secondary arc current is the current that continues to maintain the fault point after the fault-phase circuit breaker opens during SPAR operation, sustained by electromagnetic and electrostatic coupling from the healthy phases. The magnitude and characteristics of this current directly determine whether the fault arc can self-extinguish, which is critical for SPAR success.

Simulation analysis (based on EMTP, with model parameters referencing a Southern China 500 kV system) shows that installing an FCL may introduce new issues:

Impact of Bypass Switch (K) Timing: If the bypass switch K is open when the circuit breaker trips, the secondary arc current will include a component with large amplitude (up to 225 A), slow decay, and very low frequency (approximately 3–3.25 Hz). This low-frequency component significantly reduces the number of current zero-crossings, making arc self-extinction difficult and markedly lowering SPAR success rates.
Impact of Arc Path Resistance (Rg): When the transition resistance at the fault point is large (e.g., 300 Ω), the short-circuit current is small, which may prevent the FCL at the line end from activating (MOA does not reach the operating voltage). In this case, Cf remains unshort-circuited and forms a low-frequency oscillation circuit with the line shunt reactor, similarly generating a low-frequency component detrimental to arc extinction.

3. Mechanism Investigation: Origin of the Low-Frequency Component
Theoretical analysis using equivalent impedance networks and Laplace transforms reveals the mechanism behind the low-frequency component:
The root cause is capacitor Cf in the FCL. After the circuit breaker trips and the fault phase is isolated, the energy stored in Cf discharges through the shunt reactor and the fault point arc resistance. This discharge circuit forms a low-frequency oscillation circuit, with an oscillation frequency (approximately 3 Hz) primarily determined by Cf and the line shunt reactor parameters, largely independent of the fault location. This low-frequency oscillation is eliminated only when bypass switch K remains closed, completely short-circuiting Cf.

4. Core Solution: Timing Coordination Strategy for FCL and SPAR
To ensure effective current limiting by the FCL without affecting SPAR, this proposal proposes the following precise timing coordination strategy, with a total duration controlled within 0.66–0.73 seconds:

Timing Node	Time Interval (s)	Process Description
t0	-	Single-phase grounding fault occurs in the system.
t1	0.002	MOA reaches operating voltage, acts to short-circuit Cf, and current-limiting reactor L is inserted into the system.
t2	0.002	FCL monitoring system triggers the discharge gap G and simultaneously sends a signal to start closing bypass switch K.
t3	0.016	Line relay protection operates, issuing a circuit breaker trip signal, which also serves as a command to forcibly close K.
t4	≤0.024	Ensure bypass switch K is fully closed. This must be completed before the circuit breaker interrupts.
t5	0.016–0.036	Main contacts of line circuit breakers at both ends open, cutting off the fault current.
t6	0.02	Circuit breaker opening resistors disconnect, completely isolating the fault-phase line from the system; secondary arc begins to burn.
t7	0.20	During secondary arc burning, keep K closed to eliminate the low-frequency component. After arc self-extinction, issue a signal to open K.
t8	0.045	Bypass switch K opens.
t9	0.015	Fault point arc path deionization time, ensuring insulation recovery.
t10	0.10	Circuit breaker closing coil is energized, preparing for reclosing.
t11	0.20–0.25	Circuit breaker closes, with closing resistors engaged to suppress switching overvoltages.
t12	0.02	Circuit breaker main contacts close, closing resistors exit, and the line successfully resumes power supply.

Strategy Core: Use the circuit breaker trip signal from relay protection as a command to forcibly close bypass switch K quickly and keep it closed throughout the secondary arc burning period (approximately 0.2 seconds). This effectively short-circuits Cf, completely eliminating the low-frequency oscillation component in the secondary arc current and creating favorable conditions for arc self-extinction.

5. Scheme Effectiveness and Advantages
EMTP simulations verify that this timing coordination strategy achieves the following:

Eliminates Low-Frequency Harm: Completely eliminates the 3 Hz low-frequency component in the secondary arc current, avoiding its adverse effects on arc extinction.
Optimizes Arc Extinction Characteristics: Reduces secondary arc extinction time by approximately 4.5% and lowers power-frequency component current by 10.5%, significantly improving SPAR success rates.
Compatibility and Reliability: The strategy does not affect the system’s original voltage recovery characteristics and balances FCL safety (protecting the MOA) with rapid recovery needs.
Ease of Implementation: Based on existing protection signals, the strategy requires minimal modifications to secondary systems, is low-cost, and suitable for existing or new EHV projects in Southeast Asian countries.

6. Conclusion and Recommendations
For Southeast Asian EHV power grids planning or already equipped with metal-oxide arrester-type FCLs, it is essential to高度重视 the potential issue of low-frequency oscillation in secondary arc currents, which can reduce SPAR success rates and threaten power supply reliability.