Nat Geo article that explains it.

https://news.nationalgeographic.com/news/energy/2012/12/1212...

High voltage DC. High voltage means it’s more efficient (power dissipation depends on current, not voltage, so for the same power, higher voltage means much less dissipation). DC (direct current instead of alternating) makes it easier to connect disparate grids as you don’t have to worry about phasing.

With AC transmission, there are a couple of small loss terms that are not present in DC transmission. To the extent that the world surrounding the lines is conductive, there is a parasitic transformer between the transmission lines and the world. The parasitic transformer is particularly bad in salt water, which is why you see so many HVDC links crossing bodies of water. Also, the capacitive reactive current in the lines adds up over many kilometers.

HVAC distribution wins hands-down for short to medium distances, or when there are many places where you need to step up/down the voltage. It will always be cheaper to step HVAC up/down in voltage at any single point relative to DC. But HVDC transmission wins over sufficiently long distances or when crossing major bodies of water.

So I don't think we'll ever see a complete overhaul of the US distribution grid using HVDC. But we might see long-distance HVDC distribution to do things like ship solar power from the desert southwest to the west coast, or ship wind power from the Atlantic to the east coast.

I think at extreme latitudes, wind and hydro might not be available everywhere and it may be easier to do nuclear. In general I hope nuclear isn’t a big part of the solution because of cost alone: nuclear is quite expensive and I don’t really see the pathway to make it a whole lot cheaper.