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

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.

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.

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).

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.

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

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).