Why are some of the longest power lines today direct current?

Direct current lost the war of the currents, so why do some of the longest power lines today carry it? Because the old problem has a modern fix. Early low-voltage DC systems were supplanted by AC around the turn of the 20th century, since transformers made AC voltage changes practical and AC generators were more efficient. But once electronics could convert between AC and DC, DC could come back for the jobs AC does badly.

A high-voltage direct current system, or HVDC, uses direct current at 100 to 800 kilovolts. HVDC lines need fewer conductors and incur less power loss than equivalent AC lines, and, unlike AC, they can link grids that are not synchronized, even ones running at different frequencies such as 50 and 60 hertz. The latest generation of lines loses only about 1.6 percent per 1,000 km. HVDC is also especially good under the sea: AC submarine cables lose extra energy to induction and capacitance with the surrounding sea water, which HVDC avoids. The longest operational land-and-subsea interconnector is Viking Link between the UK and Denmark, at 765 km, and the longest HVDC link of all is China's Zhundong-South Anhui line, more than 3,000 km long.

There is a catch: converters at each end are expensive, so HVDC pays off only above a break-even distance, about 50 km for submarine cables and perhaps 600 to 800 km for overhead lines.

Direct current is not a perfect shield: in the 2025 Iberian blackout the HVDC lines between France and Spain ceased transmitting power as the system collapsed.