ANSI 87CU – Busbar Central Unit Protection

Description: ANSI 87CU relay provides fast and reliable busbar differential protection, ensuring fault detection and isolation in busbar systems.


πŸ”Ž What is ANSI 87CU?

The ANSI 87CU relay is a Busbar Central Unit used for differential protection in busbar systems. It detects internal faults within the busbar zone and trips the breakers to isolate the faulty section.


πŸ› οΈ Where is ANSI 87CU Used?

βœ… Busbar Protection Systems (in substations)
βœ… Main Incomer Bays (to protect incoming power sources)
βœ… Feeder Bays (to monitor outgoing circuits connected to the busbar)
βœ… Transformer Bays (when transformers are part of the busbar network)


βš™οΈ How Does It Work?

  • The 87CU relay compares the sum of currents entering and leaving the busbar.
  • Under normal conditions:
    ➑️ Incoming Current = Outgoing Current
  • During a busbar fault:
    ➑️ Incoming Current β‰  Outgoing Current
  • The relay detects this imbalance and instantly trips the associated circuit breakers to isolate the faulty section.

πŸ”‹ Real-World Example (Busbar Fault Scenario):

Imagine a substation busbar with:

  • 2 Incoming Lines (from generators or transformers)
  • 4 Outgoing Feeders (to supply different loads)

If one of the busbar connections has a short circuit, the 87CU relay will:
βœ… Detect the unbalanced current flow.
βœ… Trip only the faulty section’s breaker to maintain system stability.


πŸ” Key Benefit:

The 87CU relay ensures fast fault clearance with minimal disruption, making it essential for protecting critical power distribution systems.

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