Updated September 2026.
Almost all of the power grid runs on alternating current (AC). For moving very large amounts of power over very long distances, though, high-voltage direct current (HVDC) is often the better tool. As the best wind and solar sites are frequently far from cities, HVDC lines are becoming the backbone of the clean-energy grid.
Why DC for long distances?
- Lower losses over distance. An AC line wastes some capacity pushing “reactive” power back and forth, and the loss grows with length. A DC line doesn’t. Beyond a few hundred miles overland, HVDC usually loses less energy.
- Under water and underground. Long AC cables act like capacitors and become impractical after a few tens of miles. That’s why offshore wind farms far from shore, and long buried lines, use DC.
- Linking separate grids. HVDC can connect grids that aren’t synchronized, even ones running at different frequencies. Operators can also control exactly how much power flows.
- Narrower corridors. For the same power, DC lines need fewer conductors and smaller towers.
The downsides
- Expensive converter stations are needed at each end to switch between AC and DC. That makes HVDC uneconomic for short runs.
- Hard to tap along the way. Most lines run point to point. True multi-point DC networks need DC circuit breakers, which are still maturing.
- Permitting. In the US, the biggest obstacle isn’t technology. It’s getting approval to cross many states, counties and private properties, which can take a decade or more.
HVDC lines worth knowing
- Pacific DC Intertie (1970, upgraded since) carries power roughly 850 miles from the Pacific Northwest to Los Angeles.
- Changji–Guquan, China (2019) is the world’s first 1,100 kV line. It carries up to 12 GW across 3,300 km.
- SunZia (2026) is a 550-mile, 3 GW line carrying New Mexico wind power to Arizona and California. It went into commercial operation in 2026.
- Champlain Hudson Power Express (2026) is a fully buried 339-mile, 1,250 MW line bringing Québec hydropower to New York City. It was energized in June 2026 and is expected to supply close to a fifth of the city’s electricity.
Related: Smart Grids and Renewable Energy and Desert Power.
