---
title: "CT ratio for 33/6.6 kV star/delta (300:5)"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:differential-ct-33-6-6kv-300"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/differential-ct-33-6-6kv-300
site: BCT Engineering
---

# CT ratio for 33/6.6 kV star/delta (300:5)

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2071 Bhadra, 4 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-300
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

A three phase 33/6.6 kV star/delta connected transformer is protected by differential system. The CTs on LT side have a ratio of 300:5. Find the ratio of the CTs on the HT side.

## Short answer

With LV delta-side CTs 300/5 in star, pilot current is 5 A. HV line current is 300 x 6.6/33 = 60 A; HV CTs are in delta, so CT secondary is 5/√3 A. HV CT ratio is 60 : 5/√3 ≈ 103.9 : 5.

## Answer

In transformer differential protection the CTs on the **star** side are connected in **delta** and the CTs on the **delta** side are connected in **star**, to correct the 30° phase shift and to block zero-sequence current. Then the currents in the pilot wires from both sides must be equal.

**Data:** 33/6.6 kV, star/delta; LV (delta side) CTs 300:5, connected in star.

#### Step 1: Pilot current from LV side
Take LV line current = 300 A (CT rated current):
```text
LV CT secondary = 5 A
(star-connected, so pilot current = 5 A)
```

#### Step 2: Corresponding HV line current
```text
I_HV = 300 × 6.6/33 = 60 A
```

#### Step 3: HV CT ratio (CTs in delta)
The line current leaving a delta of CTs is √3 times the CT secondary current. For a pilot current of 5 A:
```text
CT secondary current = 5/√3 = 2.887 A
HV CT ratio = 60 : 2.887 = 103.92 : 5
```

```text
 HV (star)            LV (delta)
 CTs in delta         CTs in star
 60 A → 2.887 A       300 A → 5 A
 line out = 5 A  ──── pilots ──── 5 A
```

**Answer:** HV CT ratio = **60 : 5/√3**, i.e. about **103.9 : 5** (≈ 20.78 : 1), with HV CTs connected in delta.

## Check yourself

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

1. For a star/delta transformer under differential protection, how are the CTs connected?
   A. Star-side CTs in star, delta-side CTs in delta
   B. Star-side CTs in delta, delta-side CTs in star
   C. Both sets in star
   D. Both sets in delta
   Answer: B. Reversing the connections corrects the 30° phase shift and blocks zero-sequence current. (Key point: CT connections for star/delta transformer)

2. If the LV line current is 300 A, what is the HV line current for a 33/6.6 kV transformer?
   A. 1500 A
   B. 300 A
   C. 60 A
   D. 103.9 A
   Answer: C. I_HV = 300 × 6.6/33 = 60 A, since current is inversely proportional to voltage. (Key point: HV line current)

3. HV CTs are in delta and the pilot current must be 5 A. What is each HV CT secondary current?
   A. 5 A
   B. 5 × sqrt(3) = 8.66 A
   C. 5/sqrt(3) = 2.887 A
   D. 2.5 A
   Answer: C. The line current of a delta is sqrt(3) times the phase (CT secondary) current, so the CT secondary is 5/sqrt(3). (Key point: Delta-connected CT secondary)

4. What is the resulting HV CT ratio?
   A. 60 : 5
   B. 300 : 5
   C. 103.9 : 5
   D. 34.6 : 5
   Answer: C. 60 A corresponds to 2.887 A, so the ratio is 60 : 2.887 = 103.9 : 5. (Key point: Final HV CT ratio)

## Related in Switchgear and Protection

- [CT ratio for 33/6.6 kV star/delta (400/5)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adifferential-ct-33-6-6kv-400) — 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
- [Percentage differential relay: 125 MVA transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apercentage-differential-125mva) — Switchgear and Protection, chapter 7
- [Percentage differential relay: 150 MVA transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apercentage-differential-150mva) — 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

## Related in other subjects

- [500 kVA delta/star transformer efficiency](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Athree-phase-500kva-33-11-efficiency) — Electrical Machine, chapter 2
- [Delta/star 120 kVA transformer values](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adelta-star-120kva-line-phase-values) — Electrical Machine, chapter 2
- [Star-delta starting torque ratio and full-load slip](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aim-star-delta-starting-torque-ratio) — Electrical Machines, chapter 5
- [Star-delta transformer cannot parallel with star-star](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aparallel-star-delta-justify) — Electrical Machines, chapter 2