--- title: "Transformer protection, selection and vector group" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:transformer-protection-vector-group" source: https://www.bctengineer.com/q/semester_8/power_plant_design/transformer-protection-vector-group site: BCT Engineering --- # Transformer protection, selection and vector group Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2073 Bhadra, 8 marks - Video: [Transformer Vector Group | What is Vector Group in Transformer | Hindi](https://www.youtube.com/watch?v=z0bNwxohIwE) — Electrical With Abhishek Joshi - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/transformer-protection-vector-group - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Draw the unit protection scheme of a power transformer. Mention the selection criteria for the power transformer. What reflects the Vector group of a power transformer? ## Short answer Unit protection: percentage-biased differential (87T) with harmonic restraint. Selection criteria: MVA, voltage ratio, vector group (YNd11 for GSU), impedance 10–14%, BIL, cooling. Vector group indicates winding connection (Y, D), neutral availability, and phase displacement (e.g., 11 means LV leads HV by 30°). ## Answer #### Unit protection scheme of a power transformer The main **unit protection** of a power transformer is **percentage-biased differential protection (87T)**, backed by restricted earth fault, Buchholz and others. It compares the currents entering and leaving the transformer. Inside the zone the difference trips the relay; for through faults the currents balance. ```text HV bus Power transformer LV bus ---[CT1]-----+====( Yd11 )====+-----[CT2]--- | | Buchholz(63) | | | REF(64) | | | | +---> [ 87T bias + operate ] <---+ 2nd harmonic block (inrush), 5th (overflux) | Master trip 86 -> HV & LV CBs ``` Points to note: - CT ratios and connections (or software settings) correct for the **ratio and the phase shift** of the vector group (for Yd11, CTs on the Y side connected in delta, or numerical compensation). - **Bias (restraint)** handles CT mismatch and tap-changer range. - **2nd harmonic restraint** blocks tripping on magnetising inrush. **5th harmonic** blocking is used for overfluxing. - Supplementary protections: REF (64), Buchholz (63), oil/winding temperature (26/49), pressure relief (63PR), overcurrent and earth-fault backup (50/51, 51N), overfluxing (24). #### Selection criteria for a power transformer - **Rated MVA:** from generator rating (e.g. generator MVA plus margin), with cooling class ONAN/ONAF/OFAF. - **Voltage ratio:** generator voltage to transmission voltage (e.g. 11/132 kV); **tap changer** range (OLTC or off-circuit, e.g. ±10 % in 1.25 % or 2.5 % steps). - **Vector group** to suit the system and earthing (GSU usually YNd11 with HV star grounded). - **Impedance** (% Z): balance between fault level limit and voltage regulation/stability; usually 10–14 % for GSU. - **Number of phases:** three-phase unit or a bank of single-phase units (transport limits on hilly roads in Nepal). - **Insulation level:** BIL and power-frequency withstand. - **Losses** (no-load and load), efficiency and capitalised loss cost. - **Cooling, temperature rise, altitude** derating, noise, transport size and weight. - Standards: IEC 60076. #### What the vector group reflects The **vector group** (e.g. **YNd11, Dyn11, Yy0**) tells: 1. The **winding connection** of HV (capital letter: Y, D, Z) and LV (small letter: y, d, z). 2. Whether the **neutral is brought out** (N or n). 3. The **phase displacement** of LV with respect to HV by the **clock number**, each hour being $30^\circ$. For example "11" means LV leads HV by $30^\circ$ (11 o'clock position), and "1" means LV lags by $30^\circ$. It is essential for **parallel operation** (only transformers with the same phase shift can be paralleled), for **differential CT compensation**, and for earthing and the third-harmonic path (a delta winding traps triplen harmonics). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main unit protection of a power transformer? A. Buchholz relay only B. Percentage-biased differential protection (87T) C. Reverse power relay D. Simple overcurrent relay Answer: B. 87T compares currents entering and leaving the transformer with a bias to handle mismatch; others are supplementary. (Key point: Main unit protection) 2. Why is 2nd harmonic restraint used in the differential relay? A. To detect overfluxing B. To block tripping on magnetising inrush C. To compensate for the vector group D. To detect earth faults Answer: B. Inrush current is rich in 2nd harmonic, which is used to restrain the relay; 5th harmonic is used for overfluxing. (Key point: Harmonic restraint) 3. Which information does a vector group such as YNd11 give? A. Only the MVA rating B. Winding connections, whether the neutral is brought out, and the phase displacement by clock number C. Only the impedance D. Only the cooling type Answer: B. It shows HV and LV connections, neutral availability (N/n), and the phase shift where each hour is 30 degrees. (Key point: What the vector group reflects) ## Related in Power Plant Design - [Unit generator-transformer schemes](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aunit-generator-transformer-schemes) — Power Plant Design, chapter 4, asked 3× - [Electrical characteristics of a unit transformer](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aunit-transformer-characteristics) — Power Plant Design, chapter 4, asked 2× - [Criteria for turbine selection](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aturbine-selection-criteria) — Power Plant Design, chapter 3, asked 3× - [Protection for alternator and step-up transformer](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Amega-project-protection) — Power Plant Design, chapter 4 - [Specification for procuring a power transformer](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apower-transformer-specification) — Power Plant Design, chapter 4, asked 4× - [Quotation for a power transformer](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apower-transformer-quotation) — Power Plant Design, chapter 4 ## Related in other subjects - [Unit protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aunit-protection-scheme) — Switchgear and Protection, chapter 7 - [Differential protection of power transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Atransformer-differential-protection) — Switchgear and Protection, chapter 7 - [Converting IDMT scheme to differential protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-to-differential-transformer) — Switchgear and Protection, chapter 7 - [Basic requirements of a protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abasic-requirements-protection-scheme) — Switchgear and Protection, chapter 1, asked 3×