--- title: "Specification for procuring a power transformer" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:power-transformer-specification" source: https://www.bctengineer.com/q/semester_8/power_plant_design/power-transformer-specification site: BCT Engineering --- # Specification for procuring a power transformer Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2079 Jestha, 8 marks; 2073 Magh, 6 marks; 2070 Magh, 8 marks (asked 3 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/power-transformer-specification - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Develop a typical specification while procuring a power transformer to be used in power plant design. ## Short answer Outdoor oil-immersed 3-phase GSU to IEC 60076; e.g. 30 MVA ONAN/37.5 MVA ONAF; 132/11 kV, 50 Hz, YNd11; impedance 10–12.5%; off-circuit tap changer; HV BIL 650 kVp; ONAN/ONAF cooling; guaranteed no-load and load losses; conservator with air cell, Buchholz, PRV, OTI, WTI; routine and type tests. ## Answer A **technical specification** for a power plant's power (step-up) transformer lists the ratings, design, performance and test requirements so that bidders quote a transformer that fits the plant and grid. A typical specification (following **IEC 60076**) for a hydropower GSU is: | Item | Typical specification | |---|---| | Type, application | Outdoor, oil-immersed, 3-phase, two-winding, generator step-up | | Standard | IEC 60076 (all parts) | | Rated power | e.g. 30 MVA (ONAN) / 37.5 MVA (ONAF) | | Rated voltage ratio | e.g. 132 / 11 kV | | Frequency | 50 Hz | | Vector group | YNd11 (HV star neutral solidly earthed) | | Impedance | About 10–12.5% on rated MVA (with tolerance per IEC) | | Tap changer | Off-circuit ±2 × 2.5% on HV (or OLTC if grid needs it) | | Cooling | ONAN/ONAF (or OFWF in cavern powerhouses) | | Insulation level (BIL) | HV 650 kVp lightning impulse, 275 kV power frequency (for 145 kV class); LV 75 kVp, 28 kV | | Temperature rise | Top oil 60 K, average winding 65 K over 40 °C ambient | | Altitude | Derating/insulation correction above 1000 m | | Losses | Guaranteed no-load and load losses, capitalised in bid evaluation | | Efficiency, regulation | Stated at rated load and 0.85 pf | | Noise level | e.g. below 75 dB(A) | | Short-circuit withstand | 2 s thermal, dynamic per IEC 60076-5 | | Core and winding | CRGO core; copper windings; disc/helical type | | Oil | Mineral oil to IEC 60296 | | Bushings | Porcelain/composite, with bushing CTs for protection | | Accessories | Conservator with air-cell, silica gel breather, Buchholz relay, PRV, OTI, WTI, MOG, drain/filter valves, rollers, earthing terminals, marshalling box | | Tests | Routine (ratio, polarity, resistance, impedance, losses, dielectric), type (temperature rise, impulse) and special tests; FAT witnessed | | Transport | Max weight and dimensions for hill road access; nitrogen-filled transport if needed | | Documents and spares | Drawings, manuals, recommended spares, warranty | The rating is fixed from generator MVA (e.g. two 15 MVA units → 30 MVA), impedance from fault-level and stability studies, and BIL from the insulation coordination of the HV system. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The typical vector group for a hydropower GSU is: A. Dyn1 with LV earthed B. YNd11 with HV star neutral earthed C. Yy0 unearthed D. Dd0 Answer: B. Delta on the generator side blocks zero sequence and third harmonics; HV star neutral is earthed. (Key point: Vector group) 2. The transformer MVA rating and impedance are fixed from: A. Colour code B. Rainfall data C. Only the bidder's catalogue D. Generator MVA and fault-level/stability studies Answer: D. Rating follows generator MVA (e.g. two 15 MVA units give 30 MVA); impedance from fault-level and stability studies. (Key point: Rating basis) 3. A typical specification follows IEC 60076 and requires: A. No tests B. Only visual inspection C. Routine, type and special tests with factory acceptance tests witnessed D. Tests only after installation Answer: C. Routine, type and special tests are specified, and FAT is witnessed. (Key point: Tests and standard) ## Close variants set in other exams - [Specification for procuring a generator](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-specification) — Power Plant Design, chapter 4, asked 4× ## Related in Power Plant Design - [Design: 550 m head plant, transformer quotation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2077-550m-quotation) — Power Plant Design, chapter 4, asked 3× - [Design: 245 m head plant, standby generator](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2073-245m-standby) — Power Plant Design, chapter 4, asked 14× - [Design: 250 m head plant at Q25%](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2072-250m-q25) — Power Plant Design, chapter 4, asked 14× - [Design: 200 m head plant, bus MVA levels](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2070-200m-bus-mva) — Power Plant Design, chapter 4, asked 14× - [Design: 230 m head plant, 20 km line](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2070-230m-20km) — Power Plant Design, chapter 4, asked 14× - [Design: 250 m head plant, 66 kV double circuit](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2071-250m-0-92mh) — Power Plant Design, chapter 4, asked 13× ## Related in other subjects - [Design: 500 kVA 66/11 kV power transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adesign-500kva-66-11kv) — Electric Machine Design, chapter 3, asked 6× - [Design: 4000 kVA 66/11 kV power transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adesign-4000kva-66-11kv) — Electric Machine Design, chapter 3, asked 5× - [Typical power system network and components](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Atypical-power-system-network-components) — Power System Analysis I, chapter 1 - [Typical power system with voltage levels](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Atypical-power-system-voltage-levels) — Power System Analysis I, chapter 1