Circuit breaker protection serves as a crucial safeguard within high-voltage substations, ensuring faults are rapidly detected and isolated to maintain grid stability. Among the various configurations used, the 3/2 bus arrangement introduces unique complexities. This article explores the intricacies of breaker protection under the 3/2 scheme, encompassing aspects such as breaker failure protection, auto-reclosing, dead zone protection, charging protection, phase inconsistency, and instantaneous re-tripping.

Configuration of Breaker Protection Devices

In traditional double-bus or single-bus configurations, transmission line protection schemes typically send a tripping command to only one breaker at the local end. Consequently, automatic reclosing is also applied to that single breaker, resulting in a straightforward and logical protection structure.

In contrast, the 3/2 bus configuration demands an integrated approach. Here, breaker failure protection, auto-reclosing, phase discrepancy protection, dead zone protection, and charging protection are consolidated within a singular breaker protection device. This centralized arrangement provides comprehensive oversight over the interactions and dependencies of multiple circuit breakers connected to a shared busbar infrastructure.

Breaker Failure Protection

Breaker failure protection is imperative for ensuring rapid isolation of faults when a breaker refuses to trip after receiving a trip command from protective relays. It relies on the detection of ongoing current through the suspect breaker despite the issuance of a trip signal, allowing it to identify a malfunction and escalate action to disconnect other associated breakers. This minimizes the blackout area and averts catastrophic damage to generators, transformers, and the wider power network.

Breaker failure protection is standard for 220kV and above, and increasingly deployed at critical 110kV breakers. Under the 3/2 configuration, failure scenarios are multifaceted:

• Line Short Circuit (e.g., Line 2): If breakers 5021 and 5022 are tripped and 5021 fails, the failure protection must trip all breakers connected to the 500kV Bus I (e.g., 5011 and 5031) to extinguish the arc at the fault location.

• Bus Faults: For faults directly on the busbar, if 5021 fails to open, failure protection must trip 5022 and issue a remote trip to the opposite end of Line 2, or disengage connected transformer breakers.

• Mid-Breaker Failures: If 5022 fails during a fault on Line 2, the system must trip adjacent breakers 5021 and 5023, and send remote trip signals for the transformer or line's other ends to ensure de-energization.

This cascading logic ensures comprehensive fault clearance, even if initial breakers malfunction. If remote trip signals are not used, fault clearance depends on backup protection—often slower and less effective.

Automatic Reclosing

1. Reclosing Sequence Logic

Auto-reclosing aims to restore power after transient faults. In the 3/2 configuration, two breakers—typically a middle and side breaker—are opened for a line fault. When reclosing, both should not reclose simultaneously due to the risk of energizing a persistent fault. Instead:

• Priority to Side Breaker (e.g., 5021): If this recloses onto a fault and fails, its failure protection isolates only the Bus I breakers and remotely trips the opposite line end, minimizing impact.

• Deprioritize Mid-Breaker (e.g., 5022): If this closes first and fails, its failure protection trips adjacent transformer breakers, potentially disrupting power flows and transformer safety.

Hence, side breakers should reclose first, followed by mid-breakers only if the initial reclosure succeeds.

2. Reclosing Initiation and Mode Settings

Reclosing may be triggered by:

• Position Mismatch Start: Includes single-phase or three-phase “sneak” openings.

• External Trip Start: Triggered when protection relays issue trip commands.

Selectable modes include:

Single-phase, three-phase, composite, or disabled reclosing, chosen via local panel switches or setting control bits.

3. Reclosing Synchronization Check Methods

To prevent asynchronous closing:

Synchronism Check: Ensures line and reference voltages exceed 40V with phase angle differences within prescribed thresholds.

Dead Line Check: Verifies low voltages (<30V) and healthy voltage transformers (TVs).

No Check: Issues a close command after a fixed delay, regardless of voltage conditions.

4. Coordination Between First and Second Reclosures

The first reclose uses a shorter delay (~0.7s) and triggers a “block second reclose” signal. If the first reclose fails (fault persists), the second reclose does not proceed. If the first succeeds, the second reclose occurs with an extended delay (~1.4s). This prevents unnecessary and potentially harmful reclosures.

Charging Protection

Charging protection prevents inadvertent energization of faulty equipment during busbar or line charging. It consists of:

Two-stage phase overcurrent and one-stage zero-sequence overcurrent, all phase-sensitive.

Protection Activation: Upon excessive charging current, the respective element triggers after a preset delay, tripping the charging breaker.

Escalation: If tripping fails, breaker failure protection initiates to trip adjacent breakers.

Charging protection also locks out auto-reclosing and is only active during initial line or transformer energization, deactivating once normal conditions are confirmed.

Dead Zone Protection

Dead zones arise when faults occur between a breaker and its associated current transformer, making them invisible to protective relays. For example, a short at point K1 near the CT won’t be cleared by bus protection after the breaker opens.

Dead zone protection is designed for such invisible but critical faults:

Enabled via dedicated settings and dependent on failure protection activation.

Trigger Conditions: Simultaneous three-phase trip signals + breaker position indicators + fault current.

Trip Logic: Mirrors failure protection tripping patterns—clears all connected breakers.

It provides a faster response than failure protection, crucial for station-internal faults with high fault currents.

Three-Phase Inconsistency Protection

Three-phase inconsistency occurs when only one or two phases of a breaker operate, often due to mechanical issues, pressure faults, or auxiliary relay failures. The resulting non-full-phase operation introduces negative- and zero-sequence components, endangering equipment and misguiding protection systems.

Implementation and Requirements

Mandatory for 220kV and above under national grid reliability mandates.

Integrated within breaker body or in external breaker protection relays.

Action Logic: Detects phase mismatch using position inputs and phase current signals. If a mismatch persists (typically 2–2.5s), trips remaining phases and disables reclosing.

Auxiliary Circuit Enhancements

Advanced designs use zero-sequence and negative-sequence current logic to verify phase discrepancies, enhancing reliability.

Operational Controls

Activated when both software and hardware enable flags are present.

Inactive if phase mismatch persists for 12s, triggering an alert and locking out the function.

Protection does not trigger breaker failure logic but disables auto-reclosing upon activation.

Instantaneous Re-Tripping (Follow-Up Trip)

This optional protection enhances sensitivity and speed by immediately re-tripping under certain transient fault conditions:

Single-Phase Re-Tripping: Triggered when individual phase trips and high-current detection align.

Two-Phase Tripping with Three-Phase Reclose: Initiated with dual-phase trip signals and a high current in either.

Three-Phase Instantaneous Re-Tripping: All three-phase trip signals present along with high current detection.

Only active when initiating elements detect faults, preventing erroneous operations.

AC Voltage Disconnection Detection

Monitors potential TV (voltage transformer) failures:

Logic: If no protection is active, but total vector sum of phase voltages exceeds 12V for over 1.25s, flags abnormal TV condition.

Response: Disables synchronism and undervoltage check features of the reclose circuit, while other functions remain online.

Recovery: Automatically resets 10s after voltage normalization.

Abnormal Breaker Position Alarm

Monitors TWJ (breaker position) signals:

Triggers when:

A TWJ trip signal exists but current still flows, or

Inconsistent phase positions persist for over 10s.

An alarm is issued, indicating possible mechanical or signaling failure in the breaker assembly.

Conclusion

Breaker protection under the 3/2 bus configuration is a finely tuned interplay of logic, speed, and coordination. With multiple breakers managing a single line or busbar segment, protection schemes must be both comprehensive and adaptable. This design ensures not only fault isolation but also the integrity and continuity of power supply in the face of complex fault scenarios. Each protective element—from failure detection to dead zone clearing—functions as part of a harmonized system engineered to uphold the resilience of modern electrical grids.