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AC vs DC transmission comparison

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.

Make a detail technical and economic comparison of ac & dc transmission.

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

PointHVACHVDC
Conductors per circuit3 phases2 (bipolar) or 1 with earth return
Line reactancePresent; limits power (P=V1V2sin⁡δ/XP = V_1V_2\sin\delta / X)No reactance in steady state
Stability limitFalls with lengthNo stability limit on line length
Charging currentLarge on long lines/cablesNil
Skin effect, coronaSkin effect present; more coronaNo skin effect; lower corona loss
Reactive powerNeeds compensation for long linesConverters need 50–60% reactive support
Asynchronous linkNot possiblePossible (50/60 Hz, or unsynchronised grids)
Power flow controlIndirectFast and accurate by converter control
Fault levelInterconnection raises SCMVADoes not add fault current
HarmonicsNegligibleConverters produce harmonics; filters needed
Circuit breakingEasy (current zero)Difficult (no natural current zero)
Voltage transformationEasy with transformersOnly 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.
 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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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