--- title: "AC vs DC transmission comparison" subject: "Transmission and Distribution System Design" chapter: 1 key: "semester_8/transmission_and_distribution_system_design:ac-vs-dc-transmission" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/ac-vs-dc-transmission site: BCT Engineering --- # AC vs DC transmission comparison Transmission and Distribution System Design · Chapter 1: Introduction - Asked in: 2072 Asoj, 8 marks - Video: [Comparison between HVAC and HVDC transmission system](https://www.youtube.com/watch?v=l9nHs8e0WUg) — The Learning Curve - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/ac-vs-dc-transmission - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-1 ## Question Make a detail technical and economic comparison of ac & dc transmission. ## Short answer HVAC uses three phases, has stability limits and charging current, suits short distances economically. HVDC uses two conductors, no line reactance or stability limit, lower line cost but high terminal cost. Break-even distance is ~500–800 km overhead; HVDC is superior for long bulk transfer and cables. ## Answer Both HVAC and HVDC are used for bulk power transfer. HVAC is the normal choice; HVDC becomes better for very long distances, submarine cables and asynchronous links. #### Technical comparison | Point | HVAC | HVDC | |---|---|---| | Conductors per circuit | 3 phases | 2 (bipolar) or 1 with earth return | | Line reactance | Present; limits power ($P = V_1V_2\sin\delta / X$) | No reactance in steady state | | Stability limit | Falls with length | No stability limit on line length | | Charging current | Large on long lines/cables | Nil | | Skin effect, corona | Skin effect present; more corona | No skin effect; lower corona loss | | Reactive power | Needs compensation for long lines | Converters need 50–60% reactive support | | Asynchronous link | Not possible | Possible (50/60 Hz, or unsynchronised grids) | | Power flow control | Indirect | Fast and accurate by converter control | | Fault level | Interconnection raises SCMVA | Does not add fault current | | Harmonics | Negligible | Converters produce harmonics; filters needed | | Circuit breaking | Easy (current zero) | Difficult (no natural current zero) | | Voltage transformation | Easy with transformers | Only at converter stations | #### Economic comparison - **Line cost:** HVDC line is cheaper: fewer conductors, smaller towers, narrower right-of-way, less insulation (peak = working voltage). - **Terminal cost:** HVDC needs costly converter stations, filters, smoothing reactors and reactive support; HVAC terminals (transformers, switchgear) are cheaper. - **Losses:** HVDC has lower line loss (no skin effect, less corona, no reactive current). - **Break-even distance:** total cost lines cross at about **500–800 km for overhead lines** and **30–50 km for underground/submarine cables**. Below this HVAC is cheaper; above it HVDC is cheaper. ```text Cost | AC total | / | / DC total | / _/ | / _/ | DC_/_/ <- DC terminal cost | _/ / |/ / AC terminal cost +----|-------------------> Distance break-even (500-800 km) ``` **Conclusion of comparison:** HVAC suits short and medium distances and meshed grids with many tap points; HVDC suits long-distance bulk transfer, cables, and asynchronous interconnections. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Beyond roughly what overhead distance does HVDC become cheaper in total cost? A. 50-80 km B. 500-800 km C. 2000-3000 km D. 5-10 km Answer: B. Total cost lines cross at about 500-800 km for overhead lines (30-50 km for cables). (Key point: Break-even distance) 2. Which statement on cost is right? A. HVDC line and terminals are both cheaper B. HVDC line is cheaper but converter terminals are costly C. HVAC line is cheaper per km than HVDC D. HVAC terminals cost more than HVDC terminals Answer: B. HVDC has fewer conductors and smaller towers but needs costly converter stations, filters and reactive support. (Key point: Terminal vs line cost) 3. What can HVDC do that HVAC cannot? A. Easy voltage transformation B. Link unsynchronised grids or 50/60 Hz systems C. Break current easily at natural zero D. Avoid harmonics Answer: B. HVDC allows asynchronous interconnection since frequency need not match. (Key point: Asynchronous link) 4. Which pair is correct for HVDC compared with HVAC? A. Adds fault current; easy circuit breaking B. Does not add fault current; circuit breaking is difficult (no natural current zero) C. Adds fault current; no harmonics D. Has charging current; no skin effect Answer: B. HVDC does not raise SCMVA, but DC has no natural current zero so breaking is difficult. 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