Learn how the 87B relay protects busbar zones using differential protection for fast fault detection and selective tripping.
What is Zone Protection in 87B?
In a substation, the busbar is divided into zones for selective protection. Each zone has specific circuit breakers controlling the connected lines or transformers.
➡️ Zone 1: Covers one side of the busbar (e.g., Incomers only).
➡️ Zone 2: Covers another side of the busbar (e.g., Outgoing feeders).
➡️ Zone 3: Covers a specific section in complex busbar arrangements (e.g., double busbar systems).
The 87B relay protects these zones by detecting imbalances in current flow.
How Does 87B Zone Protection Work?
The 87B relay compares the total current entering the zone with the total current leaving the zone.
➡️ Healthy Condition: Incoming current = Outgoing current → No fault.
➡️ Fault Condition: Incoming current ≠ Outgoing current → Fault detected → Relay trips.
Example Scenario: 87B Zone Protection in a 220kV Substation
System Configuration:
- Zone 1: Includes Incomer 1 and Incomer 2 (both supplying power to the bus).
- Zone 2: Includes Feeder 1 and Feeder 2 (both drawing power from the bus).
- Each line has its own breaker.
➡️ Assume a bus fault occurs in Zone 1 (e.g., insulation failure inside the bus).
➡️ The 87B relay detects the imbalance:
- Incoming current (from Incomer 1 + Incomer 2) is greater than the outgoing current (to Feeder 1 + Feeder 2).
➡️ The relay sends a trip command to:
✅ Breaker 1 (connected to Incomer 1)
✅ Breaker 2 (connected to Incomer 2)
This isolates only the faulty zone, while other parts of the system remain energized.
Why is Zone Protection Important in 87B?
✅ Selective Tripping: Only the affected section is isolated — no unnecessary power loss.
✅ Faster Fault Clearance: Ensures quick detection and response for bus faults.
✅ System Stability: Reduces the risk of widespread blackouts.
Key Difference from Full Busbar Tripping:
- 87B Zone Protection → Trips only the affected zone’s breakers.
- BB Protection (96 BB Relay) → Trips all breakers connected to the busbar.
Real-World Example:
Imagine a 220kV busbar supplying power to 4 feeders.
➡️ A fault occurs in Zone 2 (covering Feeder 3 and Feeder 4).
➡️ The 87B relay trips only the breakers controlling Feeder 3 and Feeder 4 — other feeders continue operating normally.
This selective protection ensures maximum power availability during bus faults.
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