Direct current lost the war of the currents, then returned for long distances
Around 1900 alternating current pushed direct current out of power grids because transformers could easily step AC voltage up and down. Yet today the world's longest power line, stretching more than 3,000 kilometres across China, runs on direct current at a staggering 1,100 kilovolts.
The reason for high voltage is simple arithmetic. Power equals voltage times current, so doubling the voltage halves the current needed. Heat lost in the wires rises with the square of the current, so halving the current cuts losses by a factor of four. Fatter cables would also reduce losses, but they are heavier and dearer. Over long runs, direct current lines need fewer conductors and waste less power than equivalent AC lines.
DC has other tricks. Because its power flow can be controlled regardless of the phase relationship between the two ends, it can link AC grids that are not synchronised, or that run at different frequencies such as 50 and 60 hertz, and it can help steady a network when demand swings suddenly.
The catch was conversion. Transformers cannot handle DC, and early DC systems gave way to AC. The first long-distance transmission, in 1882 between Miesbach and Munich, used DC over 57 kilometres but carried just 1.5 kilowatts. Swiss engineer René Thury chained motor-generators in series to reach higher voltages, and his system was running in Italy by 1889. Real progress came with mercury-arc valves between 1920 and 1940. A buried cable link planned for Berlin in 1941, partly because it would be harder to bomb, was never finished; its equipment went to the Soviet Union and became the Moscow–Kashira line in 1951. A link between Gotland and mainland Sweden followed in 1954.
From the late 1960s solid-state thyristor valves took over. Canada's Eel River scheme, built by General Electric, became the first complete thyristor system in 1972, and every new HVDC project since 1977 has used solid-state devices. The last mercury-arc valves, on New Zealand's inter-island link, were switched off on 1 August 2012.
Source: High-voltage direct current
