DC Power High Voltage Transmission Advantages

DC at higher voltages has the following advantages over AC Voltage Transmission

1. DC Power can be transmitted at twice the voltage of AC. This means lower losses
   
2. DC Power uses the entire cable, where AC Power at high voltages only uses the outside due to the skin affect.
3. DC Power lines would give the ability to have a large scale grid, allowing for wind power to be used more effectively. Since different areas have wind at different times, a big grid would allow sharing to even out the supply. Same idea as having a portfolio of companies or industries to reduce the risks in stocks.
4. DC Power can be easily transmitted on the ground, where AC due to ground can't be. This also allows for underwater DC Power cables.
5. Using modern power electronics (IGBT's is my guess), Voltage conversion for DC can be done as economically as AC is done with transformers.
6. No frequencies to sync.
7. No reactance in the lines - so no coupling issues.
   

References:

• Power transmission | Where the wind blows | Economist.com
• WikiAnswers - What were the advantages and disadvantages of dc power