What is local relay protection

Local relay protection, or Local Breaker Backup (LBB) protection, ensures faults are isolated even if the primary circuit breaker fails to operate.

Overview

Local relay protection, commonly referred to as Local Breaker Backup (LBB) protection, is a critical safety mechanism in power systems designed to maintain stability and prevent equipment damage when a primary protective device, such as a circuit breaker, fails to trip during a fault . It acts as a secondary safeguard, operating independently of the primary protection system to isolate the faulted section quickly.

How LBB Protection Works

1. Fault Detection: When a fault occurs, the primary protection relay detects it and sends a trip signal to the associated circuit breaker .
2. Breaker Failure Monitoring: If the circuit breaker fails to open due to mechanical issues, trip coil failure, control circuit faults, or DC supply failure, the LBB relay monitors the fault current and breaker status .
3. Backup Action: After a short delay (typically 200–800 milliseconds), if the fault persists, the LBB relay sends trip commands to adjacent or upstream breakers to isolate the faulted section . This ensures the fault is cleared even if the original breaker cannot operate. The LBB relay continuously checks two key parameters: whether the breaker auxiliary contact is open or closed, and whether the fault current is still flowing. If the breaker opens and the fault current disappears, the LBB relay resets and waits for the next fault .

Key Features

• Independent Operation: LBB protection does not rely on communication with remote terminals, making it faster and more reliable in critical situations .
• Coverage: Unlike primary protection, which isolates only the faulted element, LBB protection may trip multiple breakers connected to the bus to ensure complete fault isolation .
• Settings: LBB relays are configured with minimum current thresholds and time delays to distinguish between normal operation and actual breaker failure .

Advantages

• Ensures system reliability by preventing cascading failures.
• Protects equipment from prolonged fault currents.
• Provides rapid backup without requiring remote communication.

Practical Example

In a substation with multiple feeders, if a fault occurs on Feeder-1 and the primary breaker fails to trip, the LBB relay will detect the continued fault current and send trip commands to other breakers (e.g., CB-1B and CB-1A) to isolate the faulty section completely, maintaining system stability .

Conclusion

Local relay protection via LBB is an essential component of modern power system protection schemes. It provides a reliable backup mechanism to ensure faults are cleared even when primary breakers fail, thereby safeguarding equipment, minimizing service interruptions, and maintaining overall system stability .