Generator choice and significance of SCR

Factors for the choice of generator

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.

Significance of SCR in generator selection

$SCR \approx 1/X_d$ (p.u.). It is specified in the purchase specification because:

• Stability: steady-state limit $P_{max} = EV\,(SCR)$; higher SCR gives a larger stability margin, important for plants connected to the grid through long lines.
• Voltage regulation: higher SCR gives less voltage change with load.
• Line charging: high SCR prevents self-excitation when charging long, unloaded lines.
• Cost and size: higher SCR needs a larger air gap and more field copper, making the machine heavier and costlier.
• Fault level: higher SCR raises short-circuit current and switchgear rating. Typical selection: hydro units about 1.0–1.2 (salient pole), turbo units 0.5–0.7.