--- title: "Meaning of infinite bus" subject: "Power System Analysis I" chapter: 1 key: "semester_4/power_system_analysis_i:infinite-bus-meaning" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/infinite-bus-meaning site: BCT Engineering --- # Meaning of infinite bus Power System Analysis I · Chapter 1: General Background - Asked in: 2076 Baisakh, 4 marks - Video: [INFINITE BUS | EASY EXPLANATION WITH DEFINITION AND DERIVATION|](https://www.youtube.com/watch?v=gPC-416sfWQ) — Notes4You - 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-meaning - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-1 ## Question What do you mean by infinite bus in power system? ## Short answer An infinite bus is an ideal bus with constant voltage magnitude and frequency, zero internal impedance, and infinite inertia. A small machine connected to it cannot change its voltage or frequency; only its own active and reactive power output adjusts. ## Answer An **infinite bus** is an ideal bus whose **voltage magnitude and frequency remain constant**, whatever real or reactive power is drawn from or injected into it. It behaves like an ideal voltage source with **zero internal impedance** and **infinite inertia**. Properties: - Voltage and frequency are fixed; the phase angle is the reference. - Thevenin impedance seen from the bus is zero ($Z_{th}\to0$, fault level $\to\infty$). - Any single machine connected to it cannot change its V or f; the machine only changes how much P and Q it exchanges. In practice, a large interconnected grid is treated as an infinite bus when the machine or load connected to it is small compared with the total system capacity. #### Explanation For a grid with Thevenin voltage $E_{th}$ and impedance $Z_{th}$, the bus voltage is $$ V = E_{th} - I Z_{th} $$ If the system is very large, $Z_{th}$ is very small (short-circuit MVA very high), so a change in current $I$ caused by one machine hardly changes $V$. Likewise, the total inertia $H$ of thousands of MW of rotating machines is so large that the power change from one machine does not change the frequency noticeably. ```text E_th --[Z_th -> 0]--+---- Infinite bus (H -> infinity) | V, f constant +--X_s--(G) small machine ``` Example: A 5 MW small hydro plant synchronised to the interconnected Nepal–India grid (hundreds of thousands of MW) sees an infinite bus. Raising its excitation only increases its reactive output; raising its turbine input only increases its active power; the grid V and f stay the same. The concept simplifies analysis of a single machine connected to a large system (stability studies, V-curves). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which describes an infinite bus? A. Voltage varies with load B. Frequency varies with load C. Voltage and frequency constant for any P and Q exchange D. Has high internal impedance Answer: C. It is an ideal voltage source with zero impedance and infinite inertia. (Key point: Infinite bus) 2. Why does the voltage stay constant when one machine changes current? A. E_th changes B. The machine controls the grid C. Current is zero D. Z_th is very small so I*Z_th drop is negligible Answer: D. V = E_th - I Z_th, with Z_th near 0. (Key point: Voltage constancy) 3. Why does frequency stay constant? A. Very large total inertia absorbs the power change from one machine B. Machine has infinite inertia C. Governor of one machine fixes it D. Frequency is not related to power balance Answer: A. Total inertia of the grid is huge. (Key point: Frequency constancy) 4. Raising turbine input of a machine on an infinite bus does what? A. Raises grid frequency B. Increases its active power only C. Raises grid voltage D. Increases reactive power only Answer: B. V and f are fixed; excitation changes Q and turbine input changes P. 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