The pacific Coast intertie runs from near The Dalles Oregon to Southern California.
If your driving around the east side of the Sierras AS in going from LA to Death Valley you will drive under it. A two line system not a multiple of 3 as most transmission systems are.
http://en.wikipedia.org/wiki/Pacific_DC_Intertie
The economics are good but I don't have all the numbers.
A big factor is the losses in the electric field. IF you stand under a 345 line sometimes you can hear the buzz. That buzz is power being dissappated into the air. The higher the voltage the greater the loss. Building and reversing that magnetic field around a transmission line can eat up a lot of power.
The inverters and converters limit the power that can be transmitted. If the power gird of Southern California were connected to The northwest grid it could in theory draw all the power out of the northwest. As lights went on in LA they would go dim in Wenatchee and Portland.
The inverter station at Sylmar acts like a 3,100 MW generator. When it reaches 3,100 MW output that's it, it can't suck more by drawing down the voltage.
They are used because they are more efficient and can limit the power transmitted.
There are a couple out of North Dakota (where there is lots of power ) over to Minnesota.
There are more out there and will be even more as windfarms are built in the Flyover Zone.
Google is your friend, look for "HVDC"