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Ultra-Fast Current Limiter (FCL): A Revolutionary Solution Featuring Millisecond-Level Interruption and Economic Benefits

RW Energy 08/26/2025

1.Overview: Redefining Speed and Economics in Short-Circuit Protection

This solution focuses on an ultra-fast short-circuit current limiting device, designed to fundamentally address the growing challenge of excessive short-circuit currents and ensure the safety of power grids and equipment.

1.1 Core Features

Ultra-Fast Interruption Speed: Detects faults and limits current within 1 millisecond, effectively restraining the short-circuit current before it reaches its prospective peak.
High Interruption Capacity:

Suitable for 12kV/17.5kV systems: Maximum breaking capacity of 210kA (RMS).
Applicable to 24kV/36kV/40.5kV systems: Maximum breaking capacity of 140kA (RMS).

1.2 Core Advantages

Economic Efficiency: Operates in parallel with current-limiting reactors to deliver the most cost-effective limiting solution. Avoids replacing entire switchgear panels or transformers due to increased short-circuit currents, significantly reducing investment in new or upgraded substations.
Broad Compatibility: Ideal for interconnecting switchgear and substations; in many scenarios (e.g., parallel operation of multiple transformers), it is the only feasible technical solution.
Exceptional Reliability:

Over 60 years of global operational experience (invented in 1955), validated in thousands of projects worldwide.
Statistics from nearly 4,000 units show an average operation frequency of only once every four years, demonstrating stable and reliable performance.

2.Key Technical Q&A

No.	Key Question	Core Answer
1	What is peak short-circuit current?	The maximum instantaneous value during the first cycle after a short-circuit fault occurs, resulting from the superposition of periodic and non-periodic components. It generates enormous electromagnetic forces (testing dynamic stability) and heat (testing thermal stability).
2	Why limit peak short-circuit current?	Peak currents exceeding equipment-rated withstand parameters can damage switchgear, circuit breakers, current transformers, and cable connectors through powerful electromagnetic forces.
3	How to adapt to parallel operation of multiple transformers?	For switchgear with a withstand capability of 2Ik, in a system with four transformers (4Ik) in parallel, perfect adaptation can be achieved by installing fast current limiters between bus sections (e.g., between sections 1-2 and 3-4).
4	What are the tripping criteria? How to avoid false trips?	The control unit simultaneously monitors instantaneous current (I) and rate of current rise (di/dt). A trip is triggered only when both exceed set thresholds. This dual criterion ensures only hazardous short-circuit currents are interrupted, while general faults are handled by downstream circuit breakers.
5	How to maintain after operation?	The core operating component (conductive bridge) features a modular design and can be returned for repair. Only the internal conductive core, inductive filler, and parallel fuses need replacement; other components are reusable, ensuring very low maintenance costs.

3.Core Functions and Value

3.1 Core Function

Detects and limits faults during the initial rising stage of short-circuit current (within 1ms), effectively preventing damage to power equipment due to insufficient dynamic and thermal stability. It perfectly compensates for the inherent limitations of traditional circuit breakers—"slow to act and unable to suppress the first half-wave peak current."

3.2 Comparative Advantages

Comparison Object	Advantage Details
Traditional Circuit Breakers	Breakers take tens of milliseconds to interrupt, unable to avoid the impact of the first peak current. This limiter responds within 1ms, restricting the actual peak short-circuit current to a lower level.
Current-Limiting Reactors	Avoids voltage drop, active losses (copper losses), and reactive losses associated with reactors in continuous operation. Also eliminates the need to address generator regulation issues caused by reactor integration.

3.3 Applicable Scenarios

Power plants
Large industrial grid substations
Specific key circuits/scenarios: Transformer/generator feeder circuits, bus tie sections, reactor bypass applications, and interconnection points between grids and captive power sources.

4.Structure and Design

4.1 Overall Composition

The three-phase AC system fast current limiter consists of:

3 conductive bridge bases
3 conductive bridges
3 matching current transformers
1 control unit

4.2 Key Component Details

Component Name	Composition / Features	Key Parameters / Rules
Conductive Bridge Base	Includes mounting plate, insulators, pulse transformer, and connectors with quick couplings	- Rated current ≥2500A and voltage 12/17.5kV: Bolted connections. - Pulse transformer: ≤17.5kV (installed only at the bottom); ≥24kV (installed at both top and bottom for reliable isolation).
Conductive Bridge	Conductive core and inductive filler encapsulated in an insulating cover	Upon tripping, the inductive filler is triggered, driving the conductive core to break rapidly at its pre-cut; current then transfers to the parallel fuse.
Matching Current Transformer	Bushing or block type, series-connected in the main circuit	Features a gapped core (high overcurrent factor, low remanence) and shielded primary/secondary windings (low impedance) to ensure measurement accuracy and speed.
Control Unit	Includes power supply, control, indication, and anti-interference units	- Dimensions: 600mm (W) × 1450mm (H) × 300mm (D); weight: 100kg. - Indication unit: 5 flag relays (3-phase trip indication + readiness monitoring + power supply monitoring).

5.Working Principle: Achieving 1ms Current Limiting

5.1 Core Composition

The device is essentially an intelligent parallel combination of two components:

"Extremely fast switch (conductive bridge)": Carries rated current during normal operation and opens instantaneously during faults.
"High-breaking-capacity fuse": Ultimately interrupts the high current after the switch opens.

5.2 Operation Sequence

Detection: Matching current transformers (CTs) continuously collect current signals; the control unit calculates instantaneous current (I) and rate of current rise (di/dt).
Judgment: When both I and di/dt exceed set values, the control unit immediately issues a trip command (independent three-phase judgment and triggering).
Interruption: The trip capacitor discharges into the pulse transformer, triggering the inductive filler in the conductive bridge. This generates high-pressure gas, causing the conductive core to rupture at its pre-cut within 1ms.
Current Limiting: Arc resistance increases rapidly, transferring current to the parallel fuse. The fuse begins limiting within 0.5ms and extinguishes the arc completely at the next current zero, clearing the fault.

5.3 Auxiliary Units

Power Unit: Provides 150V DC power to charge the trip capacitor and supply electronic components. Includes a watchdog circuit to monitor system health.
Anti-Interference Unit: All external wiring passes through this unit, providing effective protection against external electromagnetic interference and preventing false operations.

6.Commissioning and Testing

6.1 Testing Requirements

Regular functional testing is required, which can be executed by users or ABB service engineers.

6.2 Dedicated Equipment

Simulator: Temporarily replaces the conductive bridge during testing. Its built-in neon lamp lights up upon receiving a trip pulse, indicating proper operation.
Test Plug & Test Instrument: Used to check trip output voltage and overall functionality. Features a user-friendly interface and easy operation (dimensions: 400×215×320mm; weight: 11kg).

7.Scope of Supply and Parameters

7.1 Supply Models

Model Type	Applicable Scenarios	Core Configuration
Discrete Components	For installation in existing switchgear	3 bases + 3 conductive bridges + 3 CTs + 1 control unit
Drawout Cabinet	For metal-clad switchgear	Conductive bridges mounted on withdrawable carts (with isolating switch function); CTs fixed; control unit installed in the low-voltage compartment
Fixed Cabinet	- For 12/17.5/24kV systems - Mandatory for 36/40.5kV systems	All components fixed inside the cabinet. For 36/40.5kV systems, the control unit is often installed in a separate control box.

7.2 Key Technical Parameters (Example: Discrete Components)

Note: ¹ indicates forced air cooling is required; compatible with 50/60Hz frequency.

Technical Parameter	Unit	12kV	17.5kV	24kV	36/40.5kV
Rated Voltage	V	12000	17500	24000	36000/40500
Rated Current	A	1250-5000¹	1250-4000¹	2500-4000¹	1250-3000¹
Rated Short-Circuit Breaking Current (Max.)	kA RMS	210	210	210	140

8.Typical Application Scenarios

Application Scenario	Core Issue	Solution Value
Parallel System Operation	Short-circuit current from multiple transformers in parallel exceeds switchgear ratings	1. Allows reduced system impedance, minimizing voltage drop. 2. Optimizes transformer load distribution, reducing losses. 3. Enables uninterrupted load transfer during faults, improving supply reliability.
Grid-Captive Power Interconnection	Captive generator operation causes excessive short-circuit current at the common coupling point	The only rational solution. Can be equipped with directional tripping (requires CT at generator neutral) to ensure operation only for grid-side faults.
Bypassing Current-Limiting Reactors	Reactors in continuous operation cause losses and voltage drop	Bypasses reactors during normal operation (zero loss, zero voltage drop); rapidly interrupts during short circuits, diverting current to the reactor for limiting.
Selective Application of Multiple Units	Selective operation required when multiple limiters are installed on multi-section buses	Uses "current vector sum" criterion to ensure only the limiter closest to the fault operates. Supports up to 5 transformers in parallel (using 4 limiters).