📘 Breaker Failure Protection (LBB) – Working, Logic & Applications
Breaker Failure Protection ensures system safety when a circuit breaker fails to interrupt fault current. Also called Local Breaker Backup (LBB).
🔹 1️⃣ Why Breaker Failure Protection?
- Primary relay sends trip signal
- Circuit breaker must open
- If breaker fails → fault continues
- Very dangerous for system
If breaker does not clear fault → Adjacent breakers must trip
🔹 2️⃣ Working Principle
- Trip command given to breaker
- Current monitored after trip command
- If current persists beyond set time → Breaker failure declared
Time delay ≈ 100–300 ms
🔹 3️⃣ Breaker Failure Logic
- Protection relay detects fault
- Trip signal sent to breaker
- Timer starts
- If current still flowing after delay → LBB operates
- All upstream breakers trip
🔹 4️⃣ Numerical Example
Fault current = 5000 A Breaker rated interrupting time = 80 ms LBB delay setting = 150 ms
If current remains after 150 ms:
Breaker Failure Protection operates.
🔹 5️⃣ Where Used?
- 220 kV substations
- 400 kV substations
- Generator circuit breakers
- Critical transmission lines
🔹 6️⃣ Advantages
- Prevents sustained fault
- Protects equipment
- Ensures system stability
🎯 GATE & Interview Points
- Also called Local Breaker Backup (LBB)
- Operates if breaker fails to clear fault
- Uses timer + current detection
- Very important in EHV substations
Breaker Failure Protection = Protection of the Protection System
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