--- title: "Distance protection relay and applications" subject: "Switchgear and Protection" chapter: 7 key: "semester_6/switchgear_and_protection:distance-protection-relay" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/distance-protection-relay site: BCT Engineering --- # Distance protection relay and applications Switchgear and Protection · Chapter 7: Protective Relays - Asked in: 2075 Baisakh, 6 marks - Video: [Distance Protection - Introduction to Protective Relaying - Protection and Switchgear Engineering](https://www.youtube.com/watch?v=vKIk9G6kIU4) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/distance-protection-relay - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7 ## Question Explain distance protection relay. Mention their 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. ## Answer 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 | Relay | Measures | Characteristic on R–X plane | |---|---|---| | Impedance | Magnitude of Z | Circle centred at origin (non-directional) | | Reactance | X only | Straight line parallel to R axis | | Mho (admittance) | Z with direction | Circle through origin (inherently directional) | | Quadrilateral | R and X separately | Polygon (static/numerical) | #### Stepped zones ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a distance relay measure to decide whether a fault is in its zone? A. Impedance Z = V/I seen from the relay point B. Only the current magnitude C. Only the frequency D. The temperature of the line Answer: A. Line impedance is proportional to length, so measured impedance indicates distance to the fault. (Key point: Quantity measured) 2. In the balanced-beam distance relay, K1*I^2 > K2*V^2 means the relay trips when: A. V/I is greater than Z_set B. V/I is less than sqrt(K1/K2) C. V is greater than I D. I is zero Answer: B. Rearranging gives V/I < sqrt(K1/K2) = Z_set, i.e. the fault is within the set reach. (Key point: Balanced-beam operating condition) 3. Zone 1 of a distance relay is normally set to cover: A. 100% of the protected line with 1 s delay B. 80-90% of the line, instantaneous C. The whole next line, instantaneous D. 50% of the next line plus 25% beyond Answer: B. Zone 1 covers 80-90% of the line without delay; the margin avoids over-reaching beyond the remote bus. (Key point: Zone 1 setting) 4. Which relay type is preferred for long lines because it is less affected by power swings? A. Reactance B. Impedance C. Mho D. Instantaneous overcurrent Answer: C. Mho relays have a small circle through the origin, so they are less affected by power swings; reactance suits short lines. 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