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CT ratio and connection for 11/132 kV

Short answer

For delta-star transformer, LV delta-side CTs 200/1 are star connected, giving 1 A pilot. HV line current is 200 x 11/132 = 16.67 A; HV star-side CTs are delta connected, requiring secondary 1/√3 A. HV CT ratio is 16.67 : 1/√3 ≈ 28.87 : 1.

A three phase, 11/132 kV, delta-star connected power transformer is protected by differential protection. The CTs on the LV side have a current ratio of 200/1. What must be the current ratio of the CTs on the HV side? How the CTs on both the sides of the transformer are connected?

For differential protection, CTs on the delta winding side are connected in star, and CTs on the star winding side are connected in delta. This compensates the 30° phase shift of the transformer and prevents zero-sequence currents (from earth faults on the earthed star side) from causing false trips.

Data: 11/132 kV, delta (LV) / star (HV); LV CTs 200/1.

CT ratio on HV side

LV line current = 200 A → LV CT secondary (pilot) = 1 A
                (LV CTs in star)

HV line current = 200 × 11/132 = 16.67 A

HV CTs in delta: pilot current = √3 × CT secondary
CT secondary = 1/√3 = 0.5774 A

HV CT ratio = 16.67 : 0.5774 = 28.87 : 1

Connection of CTs

  • LV (11 kV, delta winding): CTs of 200/1 connected in star.
  • HV (132 kV, star winding): CTs of 16.67 : 1/√3 (≈ 28.87/1) connected in delta.
  • Pilot wires join corresponding phases of the two CT groups; the operating (and restraining) coils of the differential relays are connected between the pilots and the star point.
 11 kV delta ──[CTs 200/1, star]──┐
                                   ├─ pilots ─ 87 relays
 132 kV star ──[CTs 28.87/1, delta]┘

Answer: HV CT ratio = 16.67 : 1/√3 ≈ 28.87 : 1; LV CTs in star, HV CTs in delta.

✓ Answer checked by Claude Opus 5.5 (high effort)

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