---
title: "Define spinning reserve"
subject: "Power Plant Design"
chapter: 2
key: "semester_8/power_plant_design:define-spinning-reserve"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/define-spinning-reserve
site: BCT Engineering
---

# Define spinning reserve

Power Plant Design · Chapter 2: Integrated System Planning in Design Approach

- Asked in: 2070 Bhadra, 4 marks
- Video: [Cold Reserve, Hot Reserve, Spinning Reserve & Firm Power](https://www.youtube.com/watch?v=k_i_XwfJ3oE) — Neso Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/power_plant_design/define-spinning-reserve
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-2

## Question

Define spinning reserve (on the basis of integrated system planning approach).

## Short answer

Spinning reserve is the unused capacity of running synchronised generators, deliverable within seconds to about 10 minutes. It covers sudden loss of largest unit or load increase, and is usually sized at least equal to the largest running unit or 5-10% of load. Hydro units are ideal.

## Answer

**Spinning reserve** is the unused (unloaded) capacity of generating units that are already running and synchronised with the grid, which can be delivered quickly (within seconds to about 10 minutes) when a generator trips or load suddenly increases.

$$
\text{Spinning reserve} = \sum P_{max}(\text{units on line}) - \text{Present load}
$$

#### Purpose
- Covers the sudden loss of the largest running unit or a tie-line.
- Corrects frequency through governor action (primary and secondary control).
- Covers short-term load forecast errors.

#### Sizing rule
Usually at least equal to the **largest running unit**, or a percentage (e.g. 5–10%) of the load. It should be spread among several units, because each unit has a limited ramp rate.

#### Example
If three 50 MW hydro units are running and supplying 120 MW, the spinning reserve is $150 - 120 = 30$ MW.

In integrated planning, enough spinning reserve is kept as part of the total reserve; hydro units are ideal for it because they can change load very quickly.

## Check yourself

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

1. Spinning reserve is delivered:
   A. by units that must be started from cold
   B. by unloaded capacity of running, synchronised units within seconds to about 10 minutes
   C. only by the battery storage
   D. after hours of notice by tie-lines
   Answer: B. It is the unused capacity of units already online, quickly available when a unit trips or load jumps. (Key point: Definition)

2. Three 50 MW hydro units are running and supply 120 MW. The spinning reserve is:
   A. 120 MW
   B. 50 MW
   C. 30 MW
   D. 150 MW
   Answer: C. Spinning reserve = sum of Pmax on line - load = 150 - 120 = 30 MW. (Key point: Calculation)

3. A common sizing rule for spinning reserve is:
   A. Equal to the smallest unit
   B. At least the largest running unit, or 5-10% of load
   C. Zero if there are tie-lines
   D. Exactly equal to the peak demand
   Answer: B. It must cover loss of the largest running unit, and should be spread among several units because of limited ramp rates. (Key point: Sizing rule)

4. Why are hydro units ideal for spinning reserve?
   A. They cannot change load
   B. They can change load very quickly
   C. They need no water
   D. They have no governor
   Answer: B. Hydro units ramp load quickly, so reserve is delivered fast. (Key point: Why hydro suits)

## Related in Power Plant Design

- [Spinning reserve versus reserve power](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aspinning-reserve-vs-reserve) — Power Plant Design, chapter 2
- [Reserve planning and utility interconnection](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Areserve-planning-interconnection) — Power Plant Design, chapter 2
- [Regional interconnection and reserve bottling](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aregional-interconnection-reserve) — Power Plant Design, chapter 2
- [Power system security and reserve requirement](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asecurity-reserve-requirement) — Power Plant Design, chapter 2
- [Power system planning and grid integration](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aplanning-deregulated-grid-integration) — Power Plant Design, chapter 2
- [Power system expansion planning for a utility](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aexpansion-planning-national-utility) — Power Plant Design, chapter 2

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