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Merz-Price CT ratio for 66000/6600 V

Short answer

For the 66/6.6 kV star/delta transformer, LV CTs 400/5 in star give 5 A pilot current. HV line current is 400 x 6.6/66 = 40 A; HV CTs in delta need secondary 5/√3 A. HV CT ratio is 40 : 5/√3 ≈ 69.3 : 5.

A 3 phase 66000/6600 V star/delta transformer is protected by Merz-Price system. What will be the ratio of current transformer on the high voltage side if the current transformers on the low voltage side have a ratio of 400/5 A?

In the Merz-Price (circulating current) scheme for a star/delta transformer, the HV (star) side CTs are connected in delta and the LV (delta) side CTs in star, so that the pilot currents from both sides are equal and in phase.

Data: 66 000/6600 V, star/delta; LV CTs 400/5.

LV line current = 400 A → LV CT secondary (pilot) = 5 A

HV line current = 400 × 6600/66 000 = 40 A

HV CTs in delta: pilot current = √3 × CT secondary
CT secondary = 5/√3 = 2.887 A

HV CT ratio = 40 : 5/√3 = 40 : 2.887
            = 69.28 : 5   (≈ 13.86 : 1)

Answer: HV CT ratio = 40 : 5/√3 ≈ 69.3 : 5, with HV CTs connected in delta and LV CTs in star.

✓ Answer checked by Claude Opus 5.5 (high effort)

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