--- title: "Generator choice and significance of SCR" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:generator-choice-scr-significance" source: https://www.bctengineer.com/q/semester_8/power_plant_design/generator-choice-scr-significance site: BCT Engineering --- # Generator choice and significance of SCR Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2070 Magh, 5+3 marks - Video: [Synchronous Machine Lec- 19 | Complete Concept on Short Circuit Ratio (SCR)](https://www.youtube.com/watch?v=XlENVwD6yCQ) — EE MentorX - 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-choice-scr-significance - 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. Also mention the significance of short circuit ratio in the selection of generator. ## Short answer Factors: MVA/pf, voltage, speed/poles, shaft, reactances/SCR, inertia, efficiency, insulation class, cooling, excitation, NGT, grid requirements, transport. SCR ≈ 1/Xd sets stability: higher SCR gives larger Pmax, better voltage regulation, prevents self-excitation on long lines but increases machine size and fault level. Typical hydro SCR 1.0–1.2. ## Answer #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What fixes the speed and number of poles of a hydro generator? A. The turbine speed, through N = 120f/p B. The excitation system C. The grounding method D. The cooling type Answer: A. The generator is coupled to the turbine, so its speed fixes the pole number by N = 120f/p; hydro units are slow salient-pole machines. (Key point: Choice of speed) 2. Why is a higher SCR preferred for plants connected through long lines? A. It reduces the short-circuit current B. It gives a larger steady-state stability margin since Pmax = EV(SCR) C. It lowers the cost of the machine D. It removes the need for an AVR Answer: B. A higher SCR means a lower Xd, so Pmax and stability margin increase. (Key point: SCR and stability) 3. What is the cost of specifying a high SCR? A. A smaller and cheaper machine B. Poorer voltage regulation C. A larger air gap and more field copper, so heavier and costlier D. Lower efficiency of the turbine Answer: C. High SCR needs a larger air gap and more field copper, increasing size and cost, and it also raises fault level. (Key point: SCR cost) 4. What benefit does a high SCR give when charging a long unloaded line? A. It prevents self-excitation B. It raises the speed C. It increases the line charging current D. It lowers the fault level Answer: A. With low Xd the generator is less prone to self-excitation on capacitive loads such as long unloaded lines. (Key point: Line charging) ## 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 - [Factors in choosing a generator](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-selection-factors) — Power Plant Design, chapter 4, asked 3× - [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× - [Short circuit ratio and system stability](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ascr-significance-stability) — Power Plant Design, chapter 4, asked 3× - [Hydropower site selection factors](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asite-selection-discharge-factors) — Power Plant Design, chapter 3 - [Turbine selection method and governing factors](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aturbine-selection-method-factors) — Power Plant Design, chapter 3, asked 3× ## Related in other subjects - [Reservoir intake and site selection factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Areservoir-intake-site-factors) — Water Supply Engineering, chapter 5, asked 2× - [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 - [Distribution substation site selection factors](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Asubstation-site-location-factors) — Industrial Power Distribution and Illumination, chapter 4 - [T/F: earth wire choice depends on circuit and voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-earth-wire-choice-circuit-voltage) — Transmission and Distribution System Design, chapter 3