---
title: "Why infinite bus frequency and voltage stay constant"
subject: "Power System Analysis I"
chapter: 1
key: "semester_4/power_system_analysis_i:infinite-bus-constant-frequency"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/infinite-bus-constant-frequency
site: BCT Engineering
---

# Why infinite bus frequency and voltage stay constant

Power System Analysis I · Chapter 1: General Background

- Asked in: 2076 Bhadra, 3 marks
- Video: [#25 Operation of Synchronous Gen with Infinite bus- Introduction to House Diagram](https://www.youtube.com/watch?v=W_djVYGJLYw) — ELECTRICAL IS EASY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/infinite-bus-constant-frequency
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-1

## Question

Give reason in brief: the frequency and voltage of an infinite bus remain constant even after variation in load.

## Short answer

The voltage remains constant because the Thevenin impedance of a large grid is nearly zero, so load current changes cause negligible voltage drop. The frequency stays fixed because total inertia of all generators is so high that any load change causes only minuscule speed deviation.

## Answer

An infinite bus is a bus of a very large system (ideally zero impedance and infinite inertia), so a load change at one point is negligible compared with its capacity.

- **Voltage constant:** Bus voltage $V = E_{th} - IZ_{th}$. For a huge interconnected system, $Z_{th}\approx0$ (very high fault level), so a change in load current produces almost no voltage drop; also many generators with AVRs share the reactive demand.
- **Frequency constant:** Frequency changes when the power balance changes, at a rate set by system inertia ($df/dt \propto \Delta P/H$). The total inertia of all machines is extremely large, and many governors share any load change, so $\Delta f \approx 0$.

Hence a load change that is tiny relative to the total generation leaves the voltage and frequency practically unchanged.

## Check yourself

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

1. Why does infinite-bus voltage stay constant after a load change?
   A. Load current is zero
   B. Z_th is nearly zero so a current change gives almost no voltage drop
   C. Generators do not respond
   D. Frequency is constant
   Answer: B. V = E_th - I Z_th; with Z_th about 0, V stays put. (Key point: Voltage reason)

2. Why does frequency stay constant?
   A. Single governor controls it
   B. No inertia
   C. Huge inertia and many governors share any load change
   D. Load never changes
   Answer: C. df/dt is proportional to Delta P / H, and H is extremely large. (Key point: Frequency reason)

3. When can a system be treated as infinite bus?
   A. Load change is large
   B. System has one generator
   C. System is isolated
   D. Load change is tiny relative to total generation
   Answer: D. Only then is the change negligible. (Key point: Condition)

## Related in Power System Analysis I

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## Related in other subjects

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- [Critical clearing, fault at infinite bus, H = 6](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Acritical-clearing-fault-infinite-bus-h6) — Power System Analysis II, chapter 6
- [Power transfer and pf of motor on infinite bus](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-infinite-bus-power-transfer) — Electric Machines II, chapter 2