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Electrical characteristics of a unit transformer

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.

Discuss electrical characteristics of unit transformer that to be considered during its selection.

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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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