---
title: "Unit protection scheme"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:unit-protection-scheme"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/unit-protection-scheme
site: BCT Engineering
---

# Unit protection scheme

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2072 Asoj, 6 marks
- Video: [Differential protection](https://www.youtube.com/watch?v=ML6Dz1qHs6w) — stephen brooks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/unit-protection-scheme
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

Draw the circuit diagram of unit protection scheme and explain its operation principle and give its application.

## Short answer

Unit protection compares currents at zone boundaries via identical CTs and pilot wires; the relay carries the difference. It operates only for internal faults, giving fast, selective tripping. Used for generators, transformers, busbars, and short lines (Merz-Price differential).

## Answer

A **unit protection scheme** protects a clearly defined zone (a generator, transformer, busbar or line section) by comparing electrical quantities at all the boundaries of the zone. It operates only for faults inside its zone and is inherently selective, so it can trip instantaneously without time grading. Differential protection (Merz-Price) is the main example.

#### Circuit diagram (circulating current differential)

```text
           ┌──── Protected zone ────┐
  I₁ ──(CT1)───── equipment ─────(CT2)── I₂
         │                         │
      i₁ │      pilot wire         │ i₂
         ├─────────────────────────┤
         │           │             │
         │        [ Relay ]        │
         │     (i₁ − i₂ flows)     │
         ├─────────────────────────┤
              pilot wire
```

#### Operating principle
- Identical CTs are placed at both ends (or all terminals) of the zone, and their secondaries are connected through pilot wires so that their currents circulate.
- The relay is connected across the pilots at the equipotential point and carries the **difference** of the secondary currents (i₁ − i₂).
- **Normal load or external (through) fault:** current entering = current leaving (I₁ = I₂), so i₁ = i₂, no current flows in the relay and it does not operate, however large the through current.
- **Internal fault:** current entering ≠ current leaving (fault current is fed from one or both ends), so i₁ − i₂ flows in the relay. When it exceeds the setting the relay trips the breakers at all ends of the zone.
- To avoid false trips due to CT mismatch and saturation, **percentage bias** or high-impedance relays are used.

#### Types
- Circulating current (Merz-Price) differential.
- Balanced (opposed) voltage scheme (Translay, for feeders).
- Pilot wire and carrier (phase comparison) protection for lines.
- Restricted earth fault and frame leakage protection.

#### Applications
- **Generators:** stator phase and earth faults (biased differential).
- **Transformers:** differential protection with CTs in star/delta to correct phase shift.
- **Busbars:** high-impedance differential protection.
- **Feeders and lines:** pilot wire for short lines, carrier current/fibre-optic current differential for long lines.
- **Large motors.**

#### Merits and demerits
- Fast, sensitive and absolutely selective; no time grading needed.
- Does not give back-up for adjacent zones, so separate back-up (e.g. overcurrent) protection is required.
- Needs pilot or communication links and matched CTs.

## Check yourself

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

1. A unit protection scheme operates:
   A. For faults anywhere in the system after time grading
   B. Only for faults inside its defined zone, without time grading
   C. Only for earth faults
   D. Only as back-up
   Answer: B. It compares quantities at the zone boundaries so it is selective and instantaneous. (Key point: Unit protection meaning)

2. In the circulating current differential scheme the relay carries:
   A. i1 + i2
   B. i1 − i2
   C. i1 × i2
   D. Only i1
   Answer: B. The relay is across the pilots and carries the difference of the CT secondaries. (Key point: Relay current)

3. For a heavy through fault the relay:
   A. Trips because current is large
   B. Does not operate since i1 = i2
   C. Operates after delay
   D. Trips only one breaker
   Answer: B. Current entering equals current leaving, so the difference is zero. (Key point: External fault behaviour)

4. A drawback of unit protection is that it:
   A. Is slow
   B. Does not provide back-up for adjacent zones
   C. Needs time grading
   D. Cannot protect generators
   Answer: B. It responds only to its own zone, so separate back-up protection is needed. (Key point: Limitation)

## Related in Switchgear and Protection

- [Basic requirements of a protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abasic-requirements-protection-scheme) — Switchgear and Protection, chapter 1, asked 3×
- [Protection requirements; protective system vs scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprotective-system-vs-scheme) — Switchgear and Protection, chapter 1
- [Converting IDMT scheme to differential protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-to-differential-transformer) — Switchgear and Protection, chapter 7
- [Isolator construction, principle and application](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-construction-principle) — Switchgear and Protection, chapter 4
- [HVDC circuit breaker operation](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahvdc-circuit-breaker) — Switchgear and Protection, chapter 6
- [Vacuum circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avacuum-cb-construction) — Switchgear and Protection, chapter 6, asked 2×

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