“Finally I believe the DC transmission is done at higher voltages”
Where you could use higher voltages if you chose to, it’s very difficult and expensive and inefficient to convert high DC voltages back to AC. With AC it’s easy...
Very true. Did some googling and the only places DC transmission is implemented commercially is for undersea transmission. It also appears that the greens are trying to push it for wind farms since they generally are in the middle of nowhere and produce DC that has to be converted at some point anyway.
DC transmission lines are coming back.
It has some good advantages in certain situations that are being realized more and more.
One of the big advantages is there is no need for synchronization between different grids to transfer power back and forth.
Really long distances have less loss with DC, apparently.
http://en.wikipedia.org/wiki/High-voltage_direct_current