More on DC high voltage transmission
The use of DC avoids phase shift issues. AC current is limited to roughly 1000 miles in air, and about 5-10% of that in under water transmission (because of due to dielectric effects). Another phase problem with AC are transmission line effects, where it is possible to have twice the voltage due to reflection.
Another problem with AC is skin effect, which is roughly a quarter of an inch. Thus, DC transmission is a more efficient use of the conductor cross section.