87B – Busbar Differential Protection Relay

The 87B relay is designed to detect faults specifically within the busbar zone by applying the differential protection principle. It ensures fast and selective fault detection, which is crucial for maintaining system stability and minimizing damage.

How Does the 87B Relay Work?

The 87B relay follows the Kirchhoff’s Current Law (KCL) principle:

The sum of currents entering a node (busbar) must equal the sum of currents leaving the node.

If this condition is violated, it indicates a fault within the busbar zone.

Step-by-Step Working of the 87B Relay:

  1. Current Measurement:
    • The 87B relay monitors the current flowing through all feeders, transformers, and circuit breakers connected to the busbar.
  2. Current Summation (Differential Principle):
    • The relay calculates the total incoming current and compares it with the total outgoing current.
    • Healthy Condition: Incoming current = Outgoing current → No fault.
    • Fault Condition: Incoming current ≠ Outgoing current → Fault detected.
  3. Trip Decision:
    • If a differential current is detected (current imbalance), the 87B relay sends a trip command to the 96 BB trip relay.
    • The 96 BB relay then trips all associated breakers to isolate the faulty bus section.

Example Scenario:

Consider a 220kV busbar with the following connections:

  • Incomer 1: 500A
  • Incomer 2: 400A
  • Feeder 1: 300A
  • Feeder 2: 400A

Total Incoming Current = 500A + 400A = 900A
Total Outgoing Current = 300A + 400A = 700A

➡️ The difference (200A) means there’s a leakage or fault inside the busbar zone.
➡️ The 87B relay detects this imbalance and sends a trip signal to the 96 BB relay.
➡️ The 96 BB relay then trips all connected breakers to isolate the faulty bus section.

Why is 87B Crucial for Busbar Protection?

✅ Fast Fault Detection: Ensures quick isolation of busbar faults (within milliseconds).
✅ Selective Operation: Only trips breakers connected to the faulty section, improving system stability.
✅ Stable Protection: Immune to external disturbances like power swings or load changes.

87B + 96 BB Relay Combination:

  • The 87B relay detects the fault.
  • The 96 BB relay executes the tripping action to isolate the faulty section.
  • This combination ensures fast, selective, and secure busbar protection.

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