--- title: "Infinite bus and national grid as infinite bus" subject: "Power System Analysis I" chapter: 1 key: "semester_4/power_system_analysis_i:infinite-bus-national-grid" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/infinite-bus-national-grid site: BCT Engineering --- # Infinite bus and national grid as infinite bus Power System Analysis I · Chapter 1: General Background - Asked in: 2073 Magh, 5 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-national-grid - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-1 ## Question Describe the concept of infinite bus in electric power system. Discuss how the national electric grid of any country can be considered as an infinite bus. ## Short answer An infinite bus has fixed voltage and frequency, zero Thevenin impedance, and infinite inertia. A national grid approximates it because many generators in parallel give very low Thevenin impedance and huge stored kinetic energy, so a small machine's power or excitation changes do not noticeably alter grid voltage or frequency. ## 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. #### National grid as an infinite bus A national grid (e.g. Nepal's INPS connected with the Indian grid) links many generating stations and loads, with total capacity of thousands to hundreds of thousands of MW. - **Very low Thevenin impedance:** Many generators and lines in parallel make $Z_{th}$ very small (fault level of several thousand MVA). So $V = E_{th} - IZ_{th}$ hardly changes when one load or generator changes its current. - **Very large inertia:** The stored kinetic energy of all synchronised rotors is huge. Frequency changes as $df/dt \propto \Delta P/H_{total}$, so a change of a few MW causes negligible frequency change. - **Many controllers:** Hundreds of governors and AVRs share any change in demand, so V and f are held near nominal. Example: A 10 MW hydro plant connected to a 300,000 MW grid is only about 0.003 % of total capacity. Changing its excitation alters only its reactive output; changing its gate opening alters only its MW output; grid voltage and 50 Hz frequency remain practically constant. Hence, for that machine, the national grid acts as an infinite bus. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which describes an infinite bus? A. A bus with infinite voltage B. A bus with infinite fault resistance C. Constant voltage and frequency whatever P and Q are drawn, with zero internal impedance D. A bus whose frequency varies with load Answer: C. An infinite bus is an ideal voltage source of fixed V and f with zero Thevenin impedance and infinite inertia. (Key point: Definition of infinite bus) 2. A single machine connected to an infinite bus can change what? A. Only the P and Q it exchanges with the bus B. The bus voltage C. The bus frequency D. The bus phase reference Answer: A. The machine cannot alter V or f; it only changes its own real and reactive power exchange. (Key point: Effect of a machine on the bus) 3. Why does the voltage of a national grid hardly change when one load changes? A. Its Thevenin impedance is very large B. It has only one generator C. Loads are always constant D. Its Thevenin impedance is very small, so V = Eth - I Zth barely changes Answer: D. Many parallel generators and lines make Zth tiny, so a change in current causes negligible drop. (Key point: Why the grid is stiff in voltage) 4. What keeps grid frequency nearly constant after a few MW change? A. Absence of governors B. Zero inertia C. Very large total inertia, since df/dt is proportional to dP/H_total D. Very high line resistance Answer: C. The huge stored kinetic energy of all synchronised rotors makes df/dt negligible for small power changes. 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