--- title: "CT ratio and connection for 11/132 kV" subject: "Switchgear and Protection" chapter: 7 key: "semester_6/switchgear_and_protection:differential-ct-11-132kv" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/differential-ct-11-132kv site: BCT Engineering --- # CT ratio and connection for 11/132 kV Switchgear and Protection · Chapter 7: Protective Relays - Asked in: 2073 Bhadra, 6 marks - 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-11-132kv - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7 ## Question 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? ## 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. ## Answer 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 ```text 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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. For a delta (LV) / star (HV) transformer, how are the CTs connected? A. LV CTs star, HV CTs delta B. LV CTs delta, HV CTs star C. Both star D. Both delta Answer: A. CTs on the delta winding go in star and CTs on the star winding go in delta. (Key point: CT connections) 2. Besides phase-shift correction, what does the delta CT connection on the earthed star side prevent? A. Overvoltage on pilots B. False trip from zero-sequence current during external earth faults C. CT saturation from inrush D. Pilot capacitance effects Answer: B. Delta circulates zero-sequence current, keeping it out of the relay for external earth faults. (Key point: Why delta CTs on star side) 3. LV CTs 200/1 on 11 kV side carry 200 A. The HV (132 kV) line current is: A. 2400 A B. 16.67 A C. 200 A D. 28.87 A Answer: B. I_HV = 200 × 11/132 = 16.67 A. (Key point: HV line current) 4. With HV CTs in delta and 1 A pilot current, the HV CT ratio is: A. 16.67 : 1 B. 16.67 : sqrt(3), about 9.6 : 1 C. 16.67 : 1/sqrt(3), about 28.87 : 1 D. 200 : 1 Answer: C. CT secondary is 1/sqrt(3) = 0.5774 A, so the ratio is 16.67 : 0.5774 = 28.87 : 1. 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