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

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.

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.

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):

LV CT secondary = 5 A
(star-connected, so pilot current = 5 A)

Step 2: Corresponding HV line current

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:

CT secondary current = 5/√3 = 2.887 A
HV CT ratio = 60 : 2.887 = 103.92 : 5

 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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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