Factors in choosing a generator

The generator must match the turbine, the grid and the site. The main factors are:

1. Rating (MVA) and power factor: $S = P_{turbine}\,\eta_g / \cos\phi$; power factor usually 0.85–0.9 lagging so the unit can supply reactive power. Some overload margin (e.g. 10%) is often specified.
2. Rated voltage: usually 6.6 kV or 11 kV for hydro units (higher, e.g. 13.8–15.75 kV, for very large units); chosen with GSU and bus/cable cost.
3. Speed and number of poles: fixed by the turbine: $N = 120f/p$. Hydro units are slow, salient-pole machines with many poles.
4. Shaft arrangement: vertical (large Francis/Kaplan) or horizontal (small Pelton/Francis).
5. Reactances and SCR: $X''_d$ sets fault level and breaker rating; SCR and $X_d$ set stability and voltage regulation.
6. Inertia constant (GD²): larger inertia limits speed rise on load rejection and improves stability, but costs more.
7. Efficiency and losses: high efficiency (above 97–98%) saves energy over the plant life.
8. Insulation and temperature rise: class F insulation with class B temperature rise is common; altitude derating above 1000 m.
9. Cooling: air-cooled (open or closed with air–water coolers) for hydro.
10. Excitation system: brushless for small/medium units, static for large units.
11. Neutral grounding: high-resistance grounding via NGT.
12. Grid requirements: capability curve, reactive power range, fault ride-through, harmonics.
13. Transport and erection limits: weight and size on hill roads; split stator if needed.
14. Standards and cost: IEC 60034; capital cost versus loss capitalisation.

Example

For a 10 MW turbine output, $\eta_g = 0.97$, pf 0.85: generator rating $= 10 \times 0.97/0.85 \approx 11.4$ MVA, 11 kV, 50 Hz; if the turbine speed is 600 rpm, poles $= 120 \times 50/600 = 10$.