How Do Substations Exchange and Compare Currents in 87L Protection?

The 87L relay at each substation continuously monitors the current at its respective end of the transmission line. To compare these values, both relays exchange data via a dedicated communication link.

Step-by-Step Process:

  1. Current Measurement:
    • Each relay measures the current flowing into or out of its substation.
    • For example:
      • Station A → Measures 800A
      • Station B → Measures 700A
  2. Data Exchange via Communication Channel:
    • Each relay sends its measured current data to the relay at the other end.
    • This communication occurs through:
      ✅ Fiber Optic Cable (preferred for speed and reliability)
      ✅ Power Line Carrier (PLC) (when fiber isn’t available)
      ✅ Microwave or Ethernet (in some setups)
  3. Comparison Process:
    • Each relay now has its own current reading and the received current reading from the other end.
    • The relay compares both values.
  4. Fault Detection:
    • If the incoming and outgoing currents are equal, the system is healthy.
    • If there’s a difference, it indicates a fault inside the protected zone.
    • The relay quickly trips the breakers at both ends to isolate the fault.

Example Scenario:

Imagine a 132kV transmission line between two substations:

  • Station A measures 1000A going into the line.
  • Station B measures 950A coming out of the line.
  • The 50A difference means some current has leaked or faulted within the protected zone.
  • The 87L relay instantly detects this mismatch and trips both ends.

Key Considerations for Reliable Operation:

  1. Communication Speed:
    • Fast communication (e.g., fiber optics) ensures fault detection within milliseconds.
  2. Synchronization:
    • Both relays must operate on the same time reference (e.g., via GPS clock) to ensure accurate data comparison.
  3. Security and Stability:
    • Modern 87L relays use advanced algorithms like alpha-plane logic to distinguish between genuine faults and disturbances (e.g., CT saturation).

Summary:

The 87L relay relies on real-time communication channels and synchronization to compare current values at both ends. This ensures fast, selective, and reliable protection of transmission lines.