---
title: "CT ratio for 33/6.6 kV star/delta (400/5)"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:differential-ct-33-6-6kv-400"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/differential-ct-33-6-6kv-400
site: BCT Engineering
---

# CT ratio for 33/6.6 kV star/delta (400/5)

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2078 Chaitra, 6 marks
- Video: [Question 2 (Doubt Class) A 3-phase 33 kv/6.6 kv transformer is connected in star/delta ,ct ratio](https://www.youtube.com/watch?v=VuLVLlc-920) — Simple Electrical
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/differential-ct-33-6-6kv-400
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

A 3-phase transformer rated for 33 kV/6.6 kV is connected star/delta and the protecting current transformer on the low voltage side have a ratio of 400/5. Determine the ratio of the current transformer on the HV side.

## Short answer

With LV delta-side CTs 400/5 in star, pilot current is 5 A. HV line current is 400 x 6.6/33 = 80 A; HV CTs are in delta, so CT secondary is 5/√3 A. HV CT ratio is 80 : 5/√3 ≈ 138.6 : 5.

## Answer

For differential protection of a star/delta transformer, the CTs on the star (HV) side are connected in **delta** and those on the delta (LV) side in **star**. This corrects the 30° phase shift and keeps zero-sequence current out of the relay. The CT ratios are chosen so that equal currents flow in the pilot wires.

**Data:** 33/6.6 kV, star/delta; LV CTs 400/5 (star connected).

#### Step 1: Pilot current from LV side
Take LV line current = 400 A, so the LV CT secondary (and pilot) current = 5 A.

#### Step 2: HV line current for the same load
```text
I_HV = 400 × 6.6/33 = 80 A
```

#### Step 3: HV CT ratio
HV CTs are in delta; the line current from the delta is √3 × CT secondary current. For a pilot current of 5 A:
```text
CT secondary = 5/√3 = 2.887 A
HV CT ratio = 80 : 5/√3 = 80 : 2.887
            = 138.56 : 5   (≈ 27.71 : 1)
```

#### Connection
```text
   33 kV star ─── HV CTs (delta) ─┐
                                  ├─ pilots + relay
   6.6 kV delta ─ LV CTs (star) ──┘
```

**Answer:** HV CT ratio = **80 : 5/√3 ≈ 138.6 : 5**, HV CTs connected in delta and LV CTs in star.

## Check yourself

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

1. Why are the HV (star) CTs connected in delta in this scheme?
   A. To increase the CT output
   B. To correct the 30° phase shift and keep zero-sequence current out of the relay
   C. To reduce the burden on pilots
   D. To make the CT ratio equal to the voltage ratio
   Answer: B. Delta CTs on the star side compensate the transformer's phase shift and trap zero-sequence current. (Key point: Reason for reversed CT connections)

2. With LV current of 400 A on a 33/6.6 kV transformer, the HV line current is:
   A. 2000 A
   B. 80 A
   C. 400 A
   D. 138.6 A
   Answer: B. I_HV = 400 × 6.6/33 = 80 A. (Key point: HV line current)

3. Pilot current is 5 A and HV CTs are in delta. Which HV CT ratio results?
   A. 80 : 5
   B. 80 : 5 × sqrt(3)
   C. 80 : 5/sqrt(3), about 138.6 : 5
   D. 400 : 5
   Answer: C. CT secondary = 5/sqrt(3) = 2.887 A, so the ratio is 80 : 2.887 = 138.6 : 5. (Key point: CT ratio with delta connection)

## Related in Switchgear and Protection

- [CT ratio for 33/6.6 kV star/delta (300:5)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adifferential-ct-33-6-6kv-300) — Switchgear and Protection, chapter 7
- [CT ratio and connection for 11/132 kV](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adifferential-ct-11-132kv) — Switchgear and Protection, chapter 7
- [IDMT relays across 132/33 kV transformer (2000 A)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-132-33kv-2000a) — Switchgear and Protection, chapter 7
- [IDMT relays across 132/33 kV transformer (2700 A)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-132-33kv-2700a-graph) — Switchgear and Protection, chapter 7
- [Merz-Price CT ratio for 66000/6600 V](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Amerz-price-66kv-ct-ratio) — Switchgear and Protection, chapter 7
- [Pilot current with tap change (10 MVA)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apilot-current-10mva-tap) — Switchgear and Protection, chapter 7

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