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Impedance type distance relay

Short answer

An impedance relay uses a balanced beam with current coil (operating torque ∝ I²) and voltage coil (restraining torque ∝ V²). It trips when V/I < Z_set; its R‑X characteristic is a circle centered at origin, so a directional unit is added. Applied to medium‑length transmission lines, with stepped zones, where overcurrent grading is difficult.

Explain the working principle of impedance type distance relay and also explain its application area.

An impedance relay is a distance relay that compares the voltage and current at the relay location and operates when the ratio V/I (the impedance seen up to the fault) is less than its setting Z_s.

Working principle (balanced beam type)

   PT ── voltage coil ┐       ┌ current coil ── CT
                      ▼       ▼
        restraining ◄─[ beam ]─► operating
                       pivot
                     trip contact
  • Current coil gives operating torque ∝ I²; voltage coil gives restraining torque ∝ V².
  • Torque equation (neglecting spring): T = K₁I² − K₂V². Relay trips when K₁I² > K₂V², i.e. Z = V/I < √(K₁/K₂) = Z_s.
  • On the R–X diagram the characteristic is a circle of radius Z_s centred at the origin. Points inside the circle → trip. It is non-directional, so it is used with a directional unit.
  • Under normal load, Z is large (outside circle); during a fault within reach, Z drops inside the circle and the relay trips.

Application area

  • Protection of medium-length transmission lines, with three zones (zone 1 instantaneous for 80–90% of line).
  • Back-up protection of adjacent line sections.
  • Phase-fault protection where overcurrent grading is difficult (interconnected networks).
  • Not ideal for short lines (arc resistance error) or very long lines (power-swing tripping).

✓ Answer checked by Claude Opus 5.5 (high effort)

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