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Distance protection relay and applications

Short answer

A distance relay measures impedance V/I to detect faults within a set zone. It provides main protection for transmission lines (≥66 kV) using stepped zones: zone 1 instantaneous for 80–90% of line, zone 2 and 3 with time delays for backup. Working types include impedance, reactance, mho, and quadrilateral relays.

Explain distance protection relay. Mention their applications.

A distance relay measures the impedance (or a part of it, R or X) between the relay point and the fault, using the voltage and current at the relay: Z = V/I. Since line impedance is proportional to length, the relay operates when the measured impedance falls below the impedance of the protected section – i.e. when the fault lies within a set distance.

Principle

  • Operating torque from current, restraining torque from voltage.
  • Balanced-beam form: K₁I² > K₂V² → trips when V/I < √(K₁/K₂) = Z_set.
  • During normal load, V is high and I moderate, so Z is large → no trip. During a fault within the zone, V falls and I rises → Z small → trip.

Types

RelayMeasuresCharacteristic on R–X plane
ImpedanceMagnitude of ZCircle centred at origin (non-directional)
ReactanceX onlyStraight line parallel to R axis
Mho (admittance)Z with directionCircle through origin (inherently directional)
QuadrilateralR and X separatelyPolygon (static/numerical)

Stepped zones

A──────────B──────────C
Zone 1: 80–90% of AB  instantaneous
Zone 2: 100% AB + 20–50% BC  t ≈ 0.3–0.5 s
Zone 3: AB + BC + 25% beyond  t ≈ 1 s

Applications

  • Main protection of transmission and sub-transmission lines (66 kV and above), where grading of overcurrent relays is difficult.
  • Reactance relays for short lines (arc resistance does not affect them).
  • Impedance relays for medium lines; mho relays for long lines (less affected by power swings).
  • Back-up protection of adjacent lines (zone 2, zone 3), and protection of generators against loss of field.

Advantages: fast and selective; reach is almost independent of source impedance and fault level.

✓ Answer checked by Claude Opus 5.5 (high effort)

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