---
title: "HVDC advantages; negative polarity in homopolar link"
subject: "Power Electronics"
chapter: 7
key: "semester_7/power_electronics:hvdc-advantages-homopolar-negative"
source: https://www.bctengineer.com/q/semester_7/power_electronics/hvdc-advantages-homopolar-negative
site: BCT Engineering
---

# HVDC advantages; negative polarity in homopolar link

Power Electronics · Chapter 7: HVDC power transmission

- Asked in: 2081 Bhadra, 2+2 marks
- Video: [HVDC_UNIT-1_Introduction to HVDC Transmission _ TYPES OF HVDC LINKS](https://www.youtube.com/watch?v=5vUkvVAGxCY) — Aravind Samanu
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_electronics/hvdc-advantages-homopolar-negative
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-7

## Question

What are the advantages HVDC transmission line over the HVAC transmission line? Why negative line is used for transmission in homopolar HVDC?

## Short answer

HVDC offers fewer conductors, no stability limit, no charging current, lower losses, and asynchronous interconnection. In a homopolar link, negative polarity is used on all conductors because negative DC corona produces significantly lower power loss, radio interference, and audible noise compared to positive polarity.

## Answer

#### Advantages of HVDC over HVAC

- Only 2 conductors (bipolar) or 1 (monopolar with earth return): cheaper towers and line.
- No stability limit, so power capacity does not fall with line length.
- No reactive power and charging current; any cable length is possible.
- No skin effect; lower corona and line losses (about 2/3 of AC for the same power).
- Can connect asynchronous AC systems or systems of different frequencies.
- Fast power-flow control by firing angle; does not add to AC fault level.

#### Why negative polarity is used in homopolar HVDC

A homopolar link has two or more conductors of the same polarity with earth return. Negative polarity is chosen because:

- **Corona loss is lower** with a negative conductor than with a positive one at the same voltage.
- **Radio and audible interference** produced by corona is much less for negative polarity.
- Since all conductors have the same polarity, choosing negative gives these benefits on every conductor.

```text
  Rect ===(-Vd)=== Inv
  Rect ===(-Vd)=== Inv
   |               |
  earth ......... earth (return)
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does an HVDC line's power capacity not fall with length?
   A. It has no stability limit from reactance and load angle
   B. It uses thicker wires
   C. It has transformers along the line
   D. It has higher frequency
   Answer: A. DC has no reactance or load angle, so length does not impose a stability limit. (Key point: No stability limit)

2. What describes a homopolar HVDC link?
   A. Two conductors of opposite polarity
   B. Two or more conductors of the same polarity with earth return
   C. One conductor with no earth
   D. Three-phase conductors
   Answer: B. A homopolar link has conductors all of the same polarity and uses earth as the return path. (Key point: Homopolar link)

3. Why is negative polarity chosen for a homopolar link?
   A. It makes thyristors conduct
   B. Corona loss and radio interference are lower with a negative conductor
   C. It raises voltage
   D. Earth only accepts negative current
   Answer: B. Negative conductors produce less corona loss and radio/audible interference at the same voltage. (Key point: Why negative polarity)

4. Which capability of HVDC links does not exist with a plain AC line?
   A. Connecting AC systems of different frequencies or not in synchronism
   B. Carrying power
   C. Using transformers
   D. Having conductors
   Answer: A. DC decouples the two AC systems, so 50 Hz and 60 Hz or unsynchronised systems can be linked. (Key point: Asynchronous tie)

## Related in Power Electronics

- [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 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×
- [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
- [HVDC power loss is 2/3 of HVAC](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Ahvdc-two-thirds-loss) — Power Electronics, chapter 7, asked 2×
- [When HVDC is economical; equal transmission capacity](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Ahvdc-economical-bulk-power) — Power Electronics, chapter 7

## Related in other subjects

- [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
- [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
- [Advantages and disadvantages of HVDC over AC](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ahvdc-advantages-disadvantages) — High Voltage Engineering, chapter 1, asked 3×