--- title: "Electrical characteristics of a unit transformer" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:unit-transformer-characteristics" source: https://www.bctengineer.com/q/semester_8/power_plant_design/unit-transformer-characteristics site: BCT Engineering --- # Electrical characteristics of a unit transformer Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2074 Magh, 5 marks; 2071 Magh, 6 marks (asked 2 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/unit-transformer-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Discuss electrical characteristics of unit transformer that to be considered during its selection. ## Short answer Rated MVA ≥ generator rating; LV voltage = generator voltage, HV = grid voltage; vector group YNd11 to block zero-sequence; impedance 8–14% for fault/stability; HV taps for voltage control; low losses capitalised; insulation and temperature rise per IEC 60076; cooling ONAN/ONAF or OFWF; overfluxing capability; short-circuit withstand for 2 s. ## Answer The **unit transformer** (generator step-up transformer of a unit) connects a generator to the HV system. Its electrical characteristics must suit both the generator and the grid. #### Electrical characteristics considered 1. **Rated MVA:** at least the generator rated MVA (e.g. 15 MVA for a 12.75 MW, 0.85 pf unit); some margin for overload and the unit auxiliary load. 2. **Voltage ratio:** LV equal to generator voltage (e.g. 11 kV); HV equal to grid voltage plus margin for line drop (e.g. 132 kV). 3. **Vector group:** **YNd11** is common: delta on generator side blocks zero-sequence and third harmonics; star-earthed HV gives an earthed system. 4. **Percentage impedance:** about 8–14%. Higher impedance reduces fault level and breaker rating; lower impedance improves voltage regulation and stability. Fixed by fault-level and stability studies. 5. **Tappings:** off-circuit taps (e.g. ±2 × 2.5%) or OLTC on HV, to match grid voltage and control VAr flow. 6. **Losses and efficiency:** no-load (iron) and load (copper) losses are capitalised; low losses save energy for 30+ years. 7. **Insulation level (BIL) and temperature rise:** HV BIL as per insulation coordination; temperature rise per IEC 60076 (oil 60 K, winding 65 K); altitude correction above 1000 m. 8. **Cooling:** ONAN/ONAF for outdoor; OFWF in underground powerhouses. 9. **Overfluxing capability:** must withstand V/Hz rise on load rejection (e.g. 1.1 p.u. continuous, 1.4 p.u. for a few seconds). 10. **Short-circuit withstand:** thermal and dynamic for 2 s per IEC 60076-5. 11. **Magnetising inrush and noise level.** 12. **Standards:** IEC 60076. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is YNd11 common for a unit transformer? A. Delta on generator side blocks zero-sequence and third harmonics; earthed star HV gives an earthed system B. It reduces MVA C. It removes the need for tap changer D. It increases fault level Answer: A. The delta LV and earthed star HV combination suits generator step-up duty. (Key point: Vector group) 2. A higher percentage impedance of the transformer: A. Raises fault level B. Improves voltage regulation C. Reduces fault level and breaker rating but worsens voltage regulation and stability D. Has no effect Answer: C. Impedance trades fault level against regulation and stability, set by studies. (Key point: Percentage impedance) 3. The transformer rated MVA should be: A. Half the generator MVA B. Equal to the grid fault level C. Independent of generator D. At least the generator rated MVA with margin for overload and auxiliary load Answer: D. The transformer must carry the full generator output. 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