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HVDC advantages; negative polarity in homopolar link

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.

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

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

✓ Answer checked by Claude Opus 5.5 (high effort)

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