---
title: "Consequences of higher or lower SCR"
subject: "Power Plant Design"
chapter: 4
key: "semester_8/power_plant_design:scr-higher-lower-consequences"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/scr-higher-lower-consequences
site: BCT Engineering
---

# Consequences of higher or lower SCR

Power Plant Design · Chapter 4: Electric System Design of a Power Plant

- Asked in: 2081 Shrawan, 4 marks; 2073 Bhadra, 4 marks; 2071 Magh, 4 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/scr-higher-lower-consequences
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4

## Question

What could be the consequences of using higher or lower short circuit ratio (SCR) for a synchronous generator?

## Short answer

Higher SCR (low Xd) means larger air gap, higher cost, improved stability and voltage regulation, higher short-circuit current, and good line-charging capability. Lower SCR gives a smaller, cheaper machine but poorer stability, regulation, and fault levels. Typical values: hydro 1.0–1.5, turbo generators 0.5–0.7.

## Answer

**Short circuit ratio** $SCR \approx 1/X_d$ (p.u.). Its value changes the size, cost and behaviour of a synchronous generator.

| Aspect | Higher SCR (low $X_d$) | Lower SCR (high $X_d$) |
|---|---|---|
| Air gap | Larger | Smaller |
| Size, weight, cost | Larger, heavier, costlier | Smaller, cheaper |
| Field current and losses | Higher | Lower |
| Steady-state stability limit $EV/X_d$ | Higher, more stable | Lower, less stable |
| Synchronizing power | Higher | Lower |
| Voltage regulation | Better (less drop) | Poorer |
| Short-circuit current | Higher; heavier CBs | Lower |
| Line-charging capability | Good, resists self-excitation | Poor |
| Efficiency | Slightly lower (more field loss) | Slightly higher |

#### Consequences in brief
- **Too high SCR:** an over-sized, expensive machine with higher fault levels and more field losses; benefits may not justify the cost.
- **Too low SCR:** cheaper machine but poor stability margin and voltage regulation; needs fast excitation (static, with PSS) and careful operation; risk of losing synchronism on a weak grid.
- Typical values: hydro (salient pole) 1.0–1.5, turbo generators 0.5–0.7.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A higher SCR leads to:
   A. Smaller, cheaper machine with lower fault current
   B. Larger, costlier machine with higher fault current and better stability and regulation
   C. Poorer voltage regulation
   D. Lower stability limit
   Answer: B. High SCR (low Xd) gives better stability and regulation but a larger machine and higher fault current. (Key point: High SCR consequences)

2. A generator with too low an SCR is likely to:
   A. Have heavier circuit breakers
   B. Charge long lines easily
   C. Have better regulation
   D. Have a poorer stability margin and need fast excitation (static with PSS)
   Answer: D. Low SCR is cheaper but risks losing synchronism on a weak grid and has poor regulation. (Key point: Low SCR consequences)

3. Typical SCR values are:
   A. Hydro 1.0-1.5; turbo 0.5-0.7
   B. Hydro 0.5-0.7; turbo 1.0-1.5
   C. Both about 3
   D. Both below 0.3
   Answer: A. Salient-pole hydro machines have higher SCR than cylindrical-rotor turbo generators. (Key point: Typical values)

## 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
- [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×
- [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
- [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×

## Related in other subjects

- [Armature reaction in synchronous generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aarmature-reaction-sync-generator-loads) — Electrical Machine, chapter 6, asked 2×
- [3-phase synchronous generator construction](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Async-generator-construction-working) — Electrical Machine, chapter 6
- [Construction and operation of synchronous generator](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asynchronous-generator-construction-principle) — Electric Machines II, chapter 1
- [Synchronous generator principle and EMF equation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Agenerator-principle-emf-equation) — Electric Machines II, chapter 1