While both BB (Busbar Protection) and LBB (Local Breaker Backup) involve tripping multiple breakers, their purpose, activation conditions, and affected zones are different. Here’s a clear comparison:
Key Difference Table
| Feature | BB Protection (Busbar Protection) | LBB Protection (Local Breaker Backup) |
|---|---|---|
| Purpose | Detects faults within the busbar zone and trips all connected breakers to isolate the faulty bus section. | Provides backup protection when a breaker fails to operate during a fault. |
| Trigger Condition | Detects a differential current imbalance in the bus zone using relays like 87B. | Detects breaker failure by checking if the breaker remains closed despite receiving a trip command. |
| Fault Location | Inside the busbar zone (e.g., bus insulator flashover, conductor contact). | Outside the busbar zone (e.g., line fault or transformer fault when the breaker fails to trip). |
| Action | Trips all breakers connected to the faulty bus section. | Trips only the upstream and downstream breakers to isolate the faulty section. |
| Protection Scheme Role | Primary Protection for busbar faults. | Backup Protection for failed breaker conditions. |
| Speed | Operates very fast (within milliseconds) to minimize system disruption. | Operates with a slight time delay to allow the primary breaker time to operate first. |
| Relay Codes Used | Relays like 87B for busbar differential protection. | Relays like 96 LBB for breaker failure backup protection. |
Example Scenario 1: Busbar Protection (BB Function)
➡️ Fault Location: Inside the 220kV busbar
➡️ The 87B relay detects a differential current imbalance.
➡️ The 96 BB relay trips all breakers connected to the busbar to isolate the faulty bus section.
✅ Ensures the entire faulty bus section is isolated instantly.
Example Scenario 2: Local Breaker Backup (LBB Function)
➡️ Fault Location: On Line 1 connected to the 132kV breaker.
➡️ The primary relay (e.g., 21 Distance Relay) sends a trip signal to Breaker A.
➡️ Breaker A fails to open.
➡️ The LBB relay detects this failure and trips:
✅ The upstream incomer breaker to cut power supply.
✅ The adjacent feeder breaker to isolate the faulted line.
✅ Ensures the system remains stable even if a breaker fails.
Key Difference in Control Area:
- BB Protection → Protects the entire busbar zone.
- LBB Protection → Acts on a single breaker’s failure to isolate the fault using surrounding breakers.
Quick Summary Analogy:
- BB Protection = Fire Alarm → Detects a fire in the building and activates all sprinklers to control it.
- LBB Protection = Fire Exit Plan → If the primary fire exit is blocked (breaker failure), it directs people to alternative exits (upstream/downstream breakers) to ensure safety.