96-BB/LBB

Understanding the Setup First

In a substation, we have:

  • Busbar: Acts like a main power hub where multiple lines, transformers, and feeders are connected.
  • Circuit Breakers (CBs): Control the power flow in and out of the busbar.
  • Lines (132kV, 220kV, etc.): These are the transmission lines connected to the busbar, carrying power to different parts of the grid.

1. When a Busbar Fault Occurs → BB Protection (Busbar Protection)

➡️ A busbar fault means the main hub itself has a problem (e.g., insulation failure, conductor flashover).
➡️ Since the bus is the central hub, the BB relay (like 87B) sends a trip command to all connected breakers.
➡️ This action isolates the entire bus section — no power can flow to any connected lines until the fault is cleared.

Example:
Imagine a 220kV busbar with 5 lines connected to it. If the busbar itself gets faulty, the BB relay will trip all 5 breakers to isolate the entire bus.


**2. When a Line Fault Occurs → Primary Relay + LBB Relay (if needed)

➡️ A line fault means one of the transmission lines has an issue (e.g., short circuit on the line).
➡️ The line protection relay (like 21 Distance Relay or 87L Differential Relay) will trip the corresponding breaker.
➡️ If the breaker fails to trip, the LBB relay steps in to clear the fault by tripping:
✅ The upstream breaker (incoming power source).
✅ The adjacent breaker (next in line to isolate the faulty path).

Example:

  • Assume Line 1 (132kV) is faulty.
  • The relay sends a trip signal to Breaker A (connected to Line 1).
  • If Breaker A fails to trip, the LBB relay will:
    ✅ Trip the upstream incomer breaker to cut power supply.
    ✅ Trip the adjacent breaker to prevent fault flow.

Key Difference in Simple Terms:

  • BB Protection = Disconnects all connected breakers if the busbar itself is faulty.
  • LBB Protection = Disconnects specific breakers if a line is faulty AND the primary breaker fails.

Visual Analogy:

  • BB Protection = Imagine the main power switch in your house trips — all rooms go dark.
  • LBB Protection = If one room’s switch fails to turn off during a short circuit, the main switch + nearby room switches are tripped to isolate the problem.

Final Summary (Key Rule to Remember):

  • If the busbar itself is faulty → BB Protection trips all breakers.
  • If a line connected to the bus is faulty → Primary Relay trips the specific breaker.
  • If that breaker failsLBB Protection trips adjacent breakers to isolate the fault.

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