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Specification for procuring a power transformer
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.
Develop a typical specification while procuring a power transformer to be used in power plant design.
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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