Ron said:

>For High Voltage DC transmission systems currently in use, the monopolar
>mode (earth or sea water is the return conductor) is the most common type
>of installation.  The HV DC transmission with one wire and 'earth' is more
>efficient than AC 3-phase transmission, but the cost of the AC/DC
>conversion only makes the DC transmission have better economics if the
>line is over 500 km over land, or 20-50 km under the sea, or 40 km if all
>wires are underground.   Here are some interesting facts about this stuff.
>I don't know why it only takes 20 km underground for DC to beat out AC.
>Maybe its only one insulated wire required, so only one narrow trench?
>Any other guesses?
>

  My guess would be that it's because the $$/km for underground
  lines is typically WAY higher than overhead ones.

  Reg Neale