--- title: "Infinite bus and HVDC vs HVAC comparison" subject: "Power System Analysis I" chapter: 1 key: "semester_4/power_system_analysis_i:infinite-bus-hvdc-vs-hvac" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/infinite-bus-hvdc-vs-hvac site: BCT Engineering --- # Infinite bus and HVDC vs HVAC comparison Power System Analysis I · Chapter 1: General Background - Asked in: 2080 Chaitra, 2+4 marks - Video: [Why HIGH VOLTAGE DC power Transmission](https://www.youtube.com/watch?v=DFQG9kuXSxg) — ElectroBOOM - 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-hvdc-vs-hvac - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-1 ## Question Describe the concept of infinite bus in a power system. Compare between HVDC and HVAC systems for power delivery. ## Short answer An infinite bus has constant voltage and frequency, zero impedance, infinite inertia; a large grid approximates it. HVDC uses 1–2 conductors, no reactive power or stability limit, but expensive converters; economical beyond ~600 km. HVAC uses 3 conductors, uses transformers, but has charging current and stability limits, cheaper terminals for shorter lines. ## Answer #### Infinite bus 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. ```text [G small]--X--+ Infinite bus (V, f const) | = very large grid ``` #### HVDC vs HVAC | Basis | HVDC | HVAC | |---|---|---| | Conductors | 1 or 2 (monopole/bipole) | 3 per circuit | | Line cost | Lower (towers, conductors) | Higher | | Terminal cost | High (converters, filters) | Low (transformers) | | Break-even distance | Economical beyond ~600–800 km overhead, ~50 km cable | Economical for shorter lines | | Reactive power, charging current | None | Present; needs compensation | | Skin effect, corona | None / lower | Present / higher | | Stability limit | No stability limit on distance | Limited by $P = V_sV_r\sin\delta/X$ | | Asynchronous link | Can connect 50 Hz to 60 Hz | Needs same frequency | | Power flow control | Fast and precise | Determined by impedances | | Fault level | Not increased by link | Increases on interconnection | | Voltage transformation | Not possible directly | Easy with transformer | | Circuit breaking | Difficult (no current zero) | Easy | | Harmonics | Generated; need filters | Not a major issue | HVAC is used for most transmission and all distribution; HVDC is chosen for very long bulk transmission, submarine cables and linking asynchronous grids. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What defines an infinite bus? A. Constant current B. Infinite voltage C. Zero frequency D. Constant voltage and frequency regardless of P and Q drawn Answer: D. It is an ideal source with fixed V and f, zero internal impedance and infinite inertia. (Key point: Infinite bus definition) 2. What is the Thevenin impedance seen from an infinite bus? A. Zero, with fault level tending to infinity B. Infinite C. Equal to machine reactance D. Equal to line impedance Answer: A. Z_th -> 0 means a huge short-circuit capacity. (Key point: Thevenin impedance) 3. When is HVDC overhead transmission economical? A. Below 50 km B. Beyond roughly 600-800 km C. Only for distribution D. At any distance Answer: B. HVDC has costly terminals, so it pays only beyond the break-even distance. (Key point: Break-even distance) 4. Which capability is unique to HVDC? A. Easy voltage transformation B. Easy circuit breaking C. Connecting 50 Hz and 60 Hz systems D. Increasing fault level Answer: C. HVDC links are asynchronous; AC needs the same frequency. (Key point: Asynchronous link) 5. Why does HVAC have a distance limit that HVDC lacks? A. AC has no resistance B. DC has reactance C. HVDC has more harmonics D. AC power transfer depends on sin(delta)/X, limited by stability Answer: D. P = VsVr sin(delta)/X limits AC; DC has no power angle. (Key point: Stability limit) ## Related in Power System Analysis I - [Infinite bus and advantages of HVDC](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainfinite-bus-hvdc-advantages) — Power System Analysis I, chapter 1 - [Infinite bus and national grid as infinite bus](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainfinite-bus-national-grid) — Power System Analysis I, chapter 1 - [Economics of 1-φ vs 3-φ delivery](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aeconomics-single-vs-three-phase) — Power System Analysis I, chapter 1 - [Meaning of infinite bus](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainfinite-bus-meaning) — Power System Analysis I, chapter 1 - [Why infinite bus frequency and voltage stay constant](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainfinite-bus-constant-frequency) — Power System Analysis I, chapter 1 - [Benefits of three-phase over single-phase delivery](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Athree-phase-benefits-over-single) — Power System Analysis I, chapter 1, asked 2× ## Related in other subjects - [Compare HVAC and HVDC; 2/3 power loss](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Acompare-ac-dc-two-thirds-loss) — Power Electronics, chapter 7 - [Why HVDC over HVAC: mathematical comparison](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Ahvdc-vs-hvac-comparison) — Power Electronics, chapter 7, asked 4× - [HVDC vs HVAC line conductor saving](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Ahvdc-conductor-saving) — Power Electronics, chapter 7, asked 2× - [HVDC vs HVAC and HVDC scope in Nepal](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ahvdc-hvac-compare-nepal) — High Voltage Engineering, chapter 1