Criteria for choosing HVDC or HVAC

The choice between HVDC and HVAC is mainly economic and depends on the cost of terminals and line, the power to be transmitted and the transmission length.

1. Cost

• HVAC: terminal cost (transformers, switchgear) is low, but the line costs more per km (3 conductors, heavier towers, wider ROW, compensation).
• HVDC: terminal cost (converter stations, filters, reactive compensation) is high, but the line is cheaper per km (2 conductors, lighter towers) and losses are lower.

 Cost |          / AC
      |        /
      |      /  ___-- DC
      |    /__--
      |  _/
      |/  |
      +---+-----------------> Length
        break-even distance

2. Transmission length

• The two cost lines cross at the break-even distance: about 500–800 km for overhead lines and about 30–50 km for underground/submarine cables.
• Below this length HVAC is cheaper; above it HVDC is cheaper.
• For cables, AC charging current is so large that AC is not practical beyond about 50 km.

3. Transmission power

• HVDC terminal cost is spread over the power, so it is economical only for large power (usually above about 500–1000 MW) over long distance.
• For small power and short lines, or where power must be tapped at many points, HVAC is chosen.
• Long AC lines need series compensation as power increases, because of stability limit; DC has no such limit.

Other factors

Asynchronous interconnection, need for fast control, ROW limits and environmental issues favour HVDC; meshed networks and multiple tappings favour HVAC.