Interesting article about long-distance power lines.
◊ While it’s limited to about 800 kV, not dissimilar from AC, the way it’s constructed you effectively get double the voltage of AC.
◊ Underground and underwater lines don’t lose effectiveness for transmission compared to aboveground lines. No electromagnetic field is created by direct current to interact with other wires, the ground, or water.
◊ The wires can be arbitrarily thick because direct current doesn’t tend to flow along the surface.
◊ Direct current has no frequency, so it’s easy to connect two grids at different frequencies and use electronics to match frequency when it’s converted back to AC. HVDC is sometimes referred to as asynchronous transmission for this reason