--- title: "Factors in choosing a generator" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:generator-selection-factors" source: https://www.bctengineer.com/q/semester_8/power_plant_design/generator-selection-factors site: BCT Engineering --- # Factors in choosing a generator Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2082 Shrawan, 6 marks; 2074 Bhadra, 6 marks; 2073 Magh, 5 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/generator-selection-factors - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Discuss the various factors which should be considered for the choice of generator during the design of a power plant. ## Short answer Generator selection factors: MVA rating from turbine output and power factor (0.85–0.9), voltage (6.6–11 kV), speed/poles (N=120f/p), shaft arrangement, reactances and SCR, inertia constant, efficiency above 97%, insulation class F/B, air cooling, excitation (brushless or static), high-resistance neutral grounding, grid requirements, transport limits, and cost. ## Answer 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 $= 120 \times 50/600 = 10$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The generator speed and number of poles are: A. Chosen freely irrespective of turbine B. Set by the transformer C. Set by the cable size D. Fixed by the turbine speed using N = 120f/p Answer: D. The generator is coupled to the turbine, so poles p = 120f/N. (Key point: Speed and poles) 2. Generator MVA rating is found from: A. S = P_turbine x efficiency / power factor B. S = P_turbine x power factor C. S = P_turbine / efficiency x pf D. S = V x f Answer: A. S = P_turbine * eta_g / cos(phi); e.g. 10 MW, 0.97, 0.85 gives about 11.4 MVA. (Key point: Rating) 3. What do X''d and inertia (GD^2) influence? A. Only the paint colour B. X''d sets fault level and breaker rating; larger inertia limits speed rise on load rejection and improves stability C. X''d sets cooling; inertia sets voltage D. Nothing important Answer: B. Sub-transient reactance determines fault current; inertia helps stability but costs more. (Key point: Reactance and inertia) 4. Which excitation is usually chosen? A. Static for small units, brushless for large B. Static for all C. Brushless for small/medium units, static for large units D. Brushless for all Answer: C. Brushless is suited to small/medium units; static to large units needing fast response. (Key point: Excitation choice) ## Related in Power Plant Design - [Generator choice and SCR versus stability](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-choice-scr-stability) — Power Plant Design, chapter 4 - [Generator choice and significance of SCR](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-choice-scr-significance) — Power Plant Design, chapter 4 - [Design: 260 m head plant, generator lead](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2078-260m-generator-lead) — Power Plant Design, chapter 4, asked 5× - [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× - [Choice of generator circuit breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-circuit-breaker-choice) — Power Plant Design, chapter 4, asked 2× - [Design: 360 m head plant, 132 kV](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2078-360m-132kv) — Power Plant Design, chapter 4, asked 5× ## Related in other subjects - [Factors for choosing stations; offset types](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afactors-stations-and-offset-types) — Surveying I, chapter 3 - [Factors in power and lighting system design](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Apower-lighting-design-factors) — Industrial Power Distribution and Illumination, chapter 5 - [Firm power and plant load factors](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Afirm-power-load-factors) — Hydro Power, chapter 3 - [Factors affecting choice of BIL](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Abil-choice-factors) — Transmission and Distribution System Design, chapter 3