--- title: "Advantages and disadvantages of HVDC over AC" subject: "High Voltage Engineering" chapter: 1 key: "semester_8/high_voltage_engineering:hvdc-advantages-disadvantages" source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/hvdc-advantages-disadvantages site: BCT Engineering --- # Advantages and disadvantages of HVDC over AC High Voltage Engineering · Chapter 1: Evolution of Power System - Asked in: 2074 Bhadra; 2071 Magh, 4 marks; 2070 Bhadra, 4 marks (asked 3 times) - Video: [PS05 HVDC Transmission: Need, Advantages & Dis advantages](https://www.youtube.com/watch?v=1M8DRbsZ6Kg) — Lectures in Electrical Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/hvdc-advantages-disadvantages - Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-1 ## Question Write in brief about the advantages and disadvantages of HVDC over AC transmission. (Compare HVAC and HVDC transmission system with suitable justifications.) ## Short answer HVDC uses bipolar lines with only two conductors, eliminates stability distance limits and cable charging current, enables asynchronous grid interconnection, and offers lower losses and fast power control. However, converter stations are expensive, generate harmonics and reactive power demand, and DC breakers are difficult to build. ## Answer HVDC (High Voltage Direct Current) transmission sends bulk power as DC between two converter stations. Compared with HVAC it is cheaper and better for long lines and cables, but its converter stations are costly and complex. #### Advantages of HVDC over AC - **Fewer conductors:** a bipolar line needs only 2 conductors (3 for AC), so towers are lighter, the right-of-way is narrower and the line costs less. - **No stability limit on distance:** there is no reactance or power angle on DC, so power transfer does not fall with length. Any distance can be used. - **No charging current:** long cables (submarine or underground) can be used, while AC cables are limited to about 40–60 km. - **Lower losses:** no skin effect, no reactive current, and less corona loss for the same voltage. - **Asynchronous link:** it joins systems of different frequency (50/60 Hz) or unsynchronised grids, and does not raise their fault level. - **Fast power control:** power flow is set by the converter firing angle in milliseconds, which helps damp oscillations. - **Lower insulation level:** insulation must withstand the peak voltage. On DC the peak equals the working voltage, while on AC the peak is $\sqrt{2}$ times the rms value, so for the same power DC needs less insulation. #### Disadvantages - **Costly converter stations** (thyristor/IGBT valves, transformers, filters), so HVDC pays only beyond the break-even distance. - **Harmonics** are produced; AC and DC filters are needed. - **Reactive power demand:** line-commutated converters absorb 50–60% of the DC power as reactive power. - **DC circuit breakers** are difficult to build because DC current has no natural zero. - **No transformers on DC:** voltage cannot be stepped up or down easily, and tapping power along the route (multi-terminal) is hard. - Converters have a low overload capacity. | Point | HVAC | HVDC | |---|---|---| | Conductors | 3 | 2 (bipolar) | | Distance limit | Stability limited | None | | Terminal cost | Low | High | | Line cost | High | Low | | Cable length | 40–60 km | Very long | | Interconnection | Synchronous only | Asynchronous possible | | Reactive power | Line needs compensation | Converters need it | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a bipolar HVDC line cheaper than a three-phase AC line? A. It needs only 2 conductors, so towers are lighter and right-of-way is narrower B. It needs 4 conductors but they are thinner C. It needs no insulators because current is direct D. It uses transformers that cost less than converters Answer: A. A bipolar DC line uses 2 conductors instead of 3, giving lighter towers and narrower right-of-way. (Key point: Conductors and line cost) 2. Why can HVDC use very long cables while AC cables are limited to about 40-60 km? A. DC cables have no insulation B. DC has no charging current C. DC has a higher skin effect D. DC cable resistance is zero Answer: B. AC cables draw large capacitive charging current; DC does not. (Key point: Charging current) 3. What can HVDC do that HVAC cannot do easily? A. Step voltage up with a simple transformer B. Tap power at any point along the route C. Connect grids of different frequency or unsynchronised grids without raising fault level D. Interrupt current at its natural zero Answer: C. The DC link decouples the two systems, so frequency need not match. (Key point: Asynchronous link) 4. Why are DC circuit breakers difficult to build? A. DC voltage is always too low B. DC current is too small to detect C. DC cannot flow through switches D. DC current has no natural zero crossing Answer: D. AC current passes through zero each half cycle which helps arc extinction; DC does not. (Key point: Disadvantages) ## Related in High Voltage Engineering - [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 - [Criteria for choosing HVDC or HVAC](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ahvdc-hvac-choice-criteria) — High Voltage Engineering, chapter 1 - [Resurgence of HVDC; why AC is inevitable](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ahvdc-resurgence-ac-inevitable) — High Voltage Engineering, chapter 1 - [Issues of EHV-AC power transmission](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aehv-ac-transmission-issues) — High Voltage Engineering, chapter 1 - [HVDC for long distance and B2B significance](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ahvdc-long-distance-b2b) — High Voltage Engineering, chapter 1 - [FACTS and HVDC for Nepal's power industry](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Afacts-hvdc-nepal-upgrade) — High Voltage Engineering, chapter 1 ## 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 two conductors vs HVAC three conductors](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Ahvdc-two-vs-three-conductors) — Power Electronics, chapter 7, asked 2× - [Infinite bus and HVDC vs HVAC comparison](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainfinite-bus-hvdc-vs-hvac) — Power System Analysis I, chapter 1