---
title: "Static distance relay"
subject: "Switchgear and Protection"
chapter: 8
key: "semester_6/switchgear_and_protection:static-distance-relay"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/static-distance-relay
site: BCT Engineering
---

# Static distance relay

Switchgear and Protection · Chapter 8: Static and Digital Relays

- Asked in: 2072 Asoj, 8 marks; 2074 Bhadra, 8 marks; 2078 Chaitra, 8 marks (asked 3 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/static-distance-relay
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-8

## Question

Describe the working principle of static distance relay with a block diagram and its area of application.

## Short answer

It measures V/I using electronic comparators. Inputs: V from PT, I·Z_r from transactor; mixing circuit forms signals for mho (S1=I·Z_r−V, S2=V). Phase comparator trips when angle between S1 and S2 within ±90°. Zone timers, then amplifier/thyristor trip. Applications: EHV transmission line main/backup protection, generator loss-of-field.

## Answer

A **static distance relay** measures the impedance between the relay and the fault using electronic comparators instead of moving discs. It trips when the measured impedance Z = V/I lies inside its set characteristic (impedance, reactance, mho or quadrilateral).

#### Block diagram

```text
 PT ─►[Aux PT]──────► V ─────┐
                             ▼
                     [Mixing circuit
 CT ─►[Transactor]─► I·Zr ─►  e.g. mho: S1 = I·Zr − V,
                              S2 = V ]
                             │ S1, S2
                     [Comparator: amplitude
                       or phase type]
                             │
                     [Level detector]
                             │
                     [Zone timers T2, T3]
                             │
                     [Amplifier]─►[Output]─►Trip
```

#### Working principle

1. **Input stage:** the auxiliary PT gives a voltage proportional to the line voltage V. A **transactor** (air-gap CT) gives a voltage I·Z_r, where Z_r is the replica (set) impedance.
2. **Mixing circuit** forms two signals from V and I·Z_r. Example for a mho relay: S1 = I·Z_r − V and S2 = V.
3. **Comparator:**
   - *Amplitude comparator* (impedance relay): trips when |I·Z_r| > |V|, i.e. Z < Z_r.
   - *Phase comparator* (mho relay): trips when the angle between S1 and S2 is within ±90°, which gives a circle through the origin.
4. **Level detector** checks the comparator output against a threshold and rejects noise.
5. **Timers:** zone 1 is instantaneous; the zone 2 and zone 3 outputs pass through timers (e.g. 0.3 s and 1 s).
6. **Amplifier and output** drive the trip relay/thyristor of the CB.

Characteristics are easy to shape: by changing the mixing networks, the same hardware gives impedance, reactance, mho, offset-mho or quadrilateral characteristics.

#### Area of application

- Main protection of **EHV and HV transmission lines** (66–400 kV) with three-zone stepped protection.
- **Reactance type** for short lines (not affected by arc resistance), **mho** for long lines (least affected by power swings), and quadrilateral for lines with high fault resistance.
- **Carrier-aided schemes** (permissive/blocking) for fast clearance over the whole line.
- Back-up for adjacent lines and busbars; loss-of-field protection of generators (offset mho).
- Out-of-step blocking and power-swing detection.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the transactor (air-gap CT) supply in a static distance relay?
   A. A voltage I*Z_r proportional to current, with Z_r the replica impedance
   B. The line voltage V
   C. The DC supply
   D. The trip current
   Answer: A. The transactor gives I.Z_r which is compared with V. (Key point: Transactor)

2. An amplitude comparator impedance relay trips when:
   A. |I*Z_r| < |V|
   B. |I*Z_r| > |V|, i.e. Z < Z_r
   C. The angle exceeds 90 degrees
   D. The current is zero
   Answer: B. |I Z_r| > |V| means V/I < Z_r, the fault lies inside the set reach. (Key point: Amplitude comparator)

3. A mho relay using S1 = I*Z_r - V and S2 = V trips when:
   A. The magnitudes are equal
   B. The angle between S1 and S2 is within +/-90 degrees, giving a circle through the origin
   C. The angle is exactly 180 degrees
   D. The current exceeds the plug setting
   Answer: B. The phase comparator condition of +/-90 degrees gives the mho circle through the origin. (Key point: Phase comparator for mho)

4. Which choice matches the area of application?
   A. Reactance type for short lines, mho for long lines, main protection of EHV/HV lines
   B. Only for LV motors
   C. Only for generators
   D. Only for busbars at 11 kV
   Answer: A. Static distance relays are main protection for 66-400 kV lines; reactance for short lines (arc resistance), mho for long lines (power swings). (Key point: Application)

## Related in Switchgear and Protection

- [Static differential relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-differential-relay) — Switchgear and Protection, chapter 8, asked 3×
- [Impedance type distance relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aimpedance-distance-relay-application) — Switchgear and Protection, chapter 7
- [Static MHO relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-mho-relay) — Switchgear and Protection, chapter 8
- [Essential components of a static relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-relay-essential-components) — Switchgear and Protection, chapter 8, asked 5×
- [Directional static overcurrent relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adirectional-static-overcurrent) — Switchgear and Protection, chapter 8, asked 2×
- [Static differential relay for two-winding transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-differential-two-winding) — Switchgear and Protection, chapter 8

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