Operation and Fault Handling of High and Low Voltage Power Distribution Systems

Basic Composition and Function of Circuit Breaker Failure Protection

Circuit breaker failure protection refers to a protective scheme that operates when the relay protection of a faulty electrical device issues a trip command but the circuit breaker fails to operate. It uses the protection trip signal from the faulty equipment and the current measurement from the failed breaker to determine breaker failure. The protection can then isolate other relevant breakers within the same substation in a short time delay, minimizing the outage area, ensuring overall grid stability, preventing severe damage to generators, transformers, and other faulted components, and avoiding catastrophic grid collapse.

Breaker failure constitutes a dual fault—combining a power system fault with a breaker malfunction. While slightly relaxed performance criteria may be acceptable, the fundamental principle remains: the fault must ultimately be cleared. In modern high-voltage and extra-high-voltage power grids, breaker failure protection is widely adopted as a near-backup protection method.

Composition and Function of Failure Protection

Breaker failure protection consists of a voltage blocking element, a start-up circuit (formed by protection operation and current discrimination), a time-delay element, and a trip output circuit.

The start-up circuit is critical for the correct and reliable operation of the entire protection system. It must be secure and employ dual criteria to prevent false initiation due to single-condition judgment, stuck protection contacts, accidental contact, or unintended energization. The start-up circuit includes two elements forming an "AND" logic:

• Start Element: Typically uses the breaker’s own automatic trip output circuit. This can be the instantaneous-reset contact of the trip relay itself or a parallel auxiliary intermediate relay with instantaneous return. A contact that has operated but not reset indicates breaker failure.

• Discrimination Element: Identifies in various ways whether the fault still persists. Existing operational equipment commonly uses “current presence” methods—phase current (for lines) or zero-sequence current (for transformers). If current remains in the circuit after protection operation, it confirms the fault has not been cleared.

The time-delay element serves as an intermediate stage in breaker failure protection. To prevent maloperation caused by a single time-element failure, the time element must form an "AND" logic with the start-up circuit before activating the trip output relay.

The voltage blocking for failure protection generally consists of bus low-voltage, negative-sequence voltage, and zero-sequence voltage relays. When failure protection shares the trip output circuit with bus differential protection, they also share the same voltage blocking elements.