---
title: "IDMT settings for 20 MVA transformer and feeder"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:transformer-feeder-idmt-20mva"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/transformer-feeder-idmt-20mva
site: BCT Engineering
---

# IDMT settings for 20 MVA transformer and feeder

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2072 Asoj, 8 marks; 2079 Chaitra, 8 marks (asked 2 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/transformer-feeder-idmt-20mva
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

A 20 MVA transformer which is used to operate at 30% overload feeds an 11 kV busbar through a circuit breaker. The transformer circuit breaker is equipped with a 1000/5 CT and the feeder circuit breaker with a 400/5 CT, and both the CTs feed IDMT relays having following characteristics:

| Plug Setting Multiplier | 2 | 3 | 5 | 10 | 15 | 20 |
|---|---|---|---|---|---|---|
| Time in Second | 10 | 6 | 4.1 | 3 | 2.5 | 2.2 |

The relay on the feeder CB has 125% plug setting and a 0.3 time multiplier setting. If a fault current of 5000 A flows from the transformer to the feeder, determine: i) Operating time of feeder relay ii) Suggest suitable plug setting and time multiplier setting of the transformer relay to ensure adequate discrimination of 0.5 s between transformer and feeder relay. (The 2072 Asoj paper omits the 400/5 feeder CT ratio.)

## Short answer

Feeder: 62.5 A, PSM=10, time=0.3×3=0.9 s. Transformer: 130% overload gives 6.82 A secondary, so PS=150% (7.5 A). Fault current 25 A, PSM=3.33, time at TMS=1 is 5.68 s. Required 1.4 s, TMS=0.25. Feeder time 0.9 s, transformer PS=150%, TMS=0.25.

## Answer

The feeder relay time is found from its PSM; the transformer relay plug setting must be above the transformer overload current, and its TMS gives 0.5 s more time than the feeder relay for the same fault.

**Data:** 20 MVA, 11 kV, 30% overload; transformer CT 1000/5; feeder CT 400/5; feeder relay PS 125%, TMS 0.3; fault current 5000 A; margin 0.5 s; relay rated current 5 A.

#### i) Operating time of feeder relay
```text
CT ratio = 400/5 = 80
Relay current = 5000 / 80 = 62.5 A
Pick-up = 125% × 5 = 6.25 A
PSM = 62.5 / 6.25 = 10
Time at TMS 1 (table, PSM 10) = 3 s
Feeder relay time = 0.3 × 3 = 0.9 s
```

#### ii) Settings of transformer relay

**Plug setting:** the relay must not trip on the allowed 30% overload.

```text
Full-load current = 20 × 10⁶ / (√3 × 11 × 10³) = 1049.73 A
Max load (130%)   = 1.3 × 1049.73 = 1364.65 A
CT ratio = 1000/5 = 200
Secondary current = 1364.65 / 200 = 6.82 A
Required PS > 6.82 / 5 = 136.5%
```

Choose the next standard plug setting: **PS = 150%** (pick-up 7.5 A secondary = 1500 A primary).

**Time multiplier setting:**
```text
Relay current = 5000 / 200 = 25 A
PSM = 25 / 7.5 = 3.333
From table (linear interpolation between
PSM 3 → 6 s and PSM 5 → 4.1 s):
t = 6 − (3.333 − 3)/(5 − 3) × (6 − 4.1)
  = 6 − 0.1667 × 1.9
  = 5.683 s  (at TMS = 1)

Required time = 0.9 + 0.5 = 1.4 s
TMS = 1.4 / 5.683 = 0.2463
```

Choose **TMS = 0.25** (next practical setting). Then actual time = 0.25 × 5.683 = 1.42 s, giving a margin of 0.52 s ≥ 0.5 s.

#### Results

| Relay | CT | PS | PSM | t (TMS 1) | TMS | Time |
|---|---|---|---|---|---|---|
| Feeder | 400/5 | 125% | 10 | 3 s | 0.3 | 0.9 s |
| Transformer | 1000/5 | 150% | 3.33 | 5.68 s | 0.25 | 1.42 s |

**Answer:** feeder relay time = **0.9 s**; transformer relay **PS = 150%** and **TMS ≈ 0.25** (calculated 0.246), operating in about 1.42 s. (If the feeder CT ratio is not given, as in the 2072 paper, 400/5 is assumed.)

## Check yourself

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

1. What is the PSM of the feeder relay (CT 400/5, PS 125%, fault 5000 A)?
   A. 5
   B. 10
   C. 12.5
   D. 20
   Answer: B. Relay current = 5000/80 = 62.5 A; pick-up 6.25 A; PSM = 10, giving 3 s at TMS 1. (Key point: Feeder relay PSM)

2. What is the feeder relay operating time with TMS 0.3?
   A. 3 s
   B. 1.5 s
   C. 0.9 s
   D. 0.3 s
   Answer: C. Time = 0.3 x 3 s = 0.9 s. (Key point: Feeder relay time)

3. How is the transformer relay plug setting chosen?
   A. Below the full-load current
   B. Equal to the fault current
   C. As the lowest available setting
   D. Above the 130% overload current in secondary amps (>136.5%), so choose 150%
   Answer: D. Max load is 1364.65 A, secondary 6.82 A, so PS must exceed 136.5% and the next standard value of 150% is used. (Key point: Transformer plug setting)

4. What operating time is required of the transformer relay, and what is the resulting TMS (about)?
   A. 1.4 s, TMS about 0.25
   B. 0.9 s, TMS about 0.16
   C. 0.5 s, TMS about 0.09
   D. 1.4 s, TMS about 0.5
   Answer: A. Required time = 0.9 + 0.5 = 1.4 s; at PSM 3.33 the TMS 1 time is 5.68 s, so TMS = 1.4/5.68 = 0.246, set to 0.25. (Key point: Transformer TMS)

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