Agreed. There's two things about HVDC worth mentioning:
It is High-Voltage because high voltage is efficient for transmission. On a first-order approximation, resistive losses are proportional to current, not power. Since P=VA, the resistive losses decrease for a given power as more of the power shifts from the A to the V.
Second, a neat property of DC transmission is it allows phase-decoupling between grids. For you to get a clean 120V, 60Hz power line at your outlet, all the transmission and distribution infrastructure needs to be tightly balanced. When that 60Hz starts to deviate by like 0.01%, alarm bells start going off at the utility. Now imagine keeping that AC stuff sync'd on a state-wide transmission line. If you make the line DC instead, you can transmit power willy-nilly across the length of the line and just make sure it syncs up at the ends when you change it back to AC.
We haven't traditionally used DC because it has higher transmission losses than AC until you get up to the ultra-high voltages (that's famously why Edison's AC system won out to Tesla's DC in NYC back in the day). My understanding is it's just been really hard to get up to the ultra-high voltages needed to make it work. 12V DC transmission barely works at even house-scale because of the resistive losses, but when you get up to 1MV like they're building in China, it starts to get attractive, especially when linking different sub-grids half a continent apart.
Volt for volt, doesn't AC have higher transmission losses than DC because of the skin effect? I was under the impression AC was what we chose for the grid because transformers make it cheap and easy to step up and step down voltage.
Agreed. There's two things about HVDC worth mentioning:
It is High-Voltage because high voltage is efficient for transmission. On a first-order approximation, resistive losses are proportional to current, not power. Since P=VA, the resistive losses decrease for a given power as more of the power shifts from the A to the V.
Second, a neat property of DC transmission is it allows phase-decoupling between grids. For you to get a clean 120V, 60Hz power line at your outlet, all the transmission and distribution infrastructure needs to be tightly balanced. When that 60Hz starts to deviate by like 0.01%, alarm bells start going off at the utility. Now imagine keeping that AC stuff sync'd on a state-wide transmission line. If you make the line DC instead, you can transmit power willy-nilly across the length of the line and just make sure it syncs up at the ends when you change it back to AC.
We haven't traditionally used DC because it has higher transmission losses than AC until you get up to the ultra-high voltages (that's famously why Edison's AC system won out to Tesla's DC in NYC back in the day). My understanding is it's just been really hard to get up to the ultra-high voltages needed to make it work. 12V DC transmission barely works at even house-scale because of the resistive losses, but when you get up to 1MV like they're building in China, it starts to get attractive, especially when linking different sub-grids half a continent apart.