---
title: "Power system security and reserve requirement"
subject: "Power Plant Design"
chapter: 2
key: "semester_8/power_plant_design:security-reserve-requirement"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/security-reserve-requirement
site: BCT Engineering
---

# Power system security and reserve requirement

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

- Asked in: 2081 Shrawan, 4 marks
- Video: [General Concept of Operating Reserve - Operating Reserve - Power System Planning and Reliability](https://www.youtube.com/watch?v=UMu3wBsX7EA) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/power_plant_design/security-reserve-requirement
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-2

## Question

What is power system security? Also describe about reserve requirement in a power system.

## Short answer

Power system security is the ability to withstand sudden disturbances and continue supplying load, usually meeting the N-1 criterion. Reserve is capacity above peak demand, comprising spinning, non-spinning, and planning reserves; the reserve margin is typically 15-25%.

## Answer

**Power system security** is the ability of the power system to withstand sudden disturbances such as loss of a generator, line or transformer, or a short circuit, and continue supplying load without violating voltage, frequency and loading limits. The usual criterion is **N-1**: the system must remain secure after any single outage.

#### Reserve requirement
**Reserve** is the generating capacity kept above the forecast peak demand.

- **Spinning reserve:** unloaded capacity of synchronised units; at least equal to the largest running unit, for sudden outages and frequency control.
- **Non-spinning (standby) reserve:** quick-start units (hydro, gas turbine, diesel) that can come on line within 10–30 minutes.
- **Planning (cold) reserve:** extra installed capacity for maintenance, forced outages, load growth and dry-year hydrology.

**Reserve margin**

$$
RM = \frac{P_{installed} - P_{peak}}{P_{peak}} \times 100\%
$$

Typical values are 15–25%. It is fixed either deterministically (largest unit, fixed %) or probabilistically (LOLP/LOLE target). Interconnection with neighbouring grids reduces the reserve requirement.

## Check yourself

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

1. Power system security is the ability to:
   A. Prevent all faults from occurring
   B. Withstand sudden disturbances and keep supplying load within voltage, frequency and loading limits
   C. Generate more than the demand always
   D. Protect the system from theft
   Answer: B. A secure system survives contingencies without violating limits. (Key point: Power system security)

2. The N-1 criterion requires that:
   A. The system is secure after any single outage
   B. The system survives only when one unit is left
   C. Every unit has one spare
   D. Only the largest line is checked
   Answer: A. N-1 means the system stays secure after the loss of any one element. (Key point: N-1 criterion)

3. Which is correctly matched?
   A. Spinning: quick-start units that are off-line
   B. Non-spinning: unloaded capacity of synchronised units
   C. Planning (cold): extra installed capacity for maintenance, forced outages, growth and dry years
   D. Planning: only frequency control
   Answer: C. Planning/cold reserve is extra installed capacity; spinning is online unloaded capacity; non-spinning is quick-start units. (Key point: Types of reserve)

4. Reserve margin is calculated as:
   A. (P_peak - P_installed)/P_installed
   B. (P_installed - P_peak)/P_peak x 100%
   C. P_peak/P_installed
   D. P_installed x P_peak
   Answer: B. RM = (installed - peak)/peak x 100%, typically 15-25%. (Key point: Reserve margin)

## Related in Power Plant Design

- [Concept of power system security](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apower-system-security-concept) — Power Plant Design, chapter 2, asked 3×
- [Power system security assessment](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asecurity-assessment-components) — Power Plant Design, chapter 2
- [Accurate load forecasting and system security](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aload-forecasting-benefits-security) — Power Plant Design, chapter 2
- [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
- [Define reserve power](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adefine-reserve-power) — 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

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- [Fire fighting system in power station](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Afire-fighting-power-station) — Power Plant Equipment, chapter 4, asked 5×
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