High-Voltage Direct Current

High-voltage direct current, known as HVDC, is an electric power transmission technology that moves power as direct current at voltages typically between 100 and 800 kilovolts, rather than the alternating current used by most transmission grids. An HVDC link uses fewer conductors and suffers substantially lower transmission losses than an equivalent AC line, roughly 3.5 percent per 1,000 kilometers compared with about 6.7 percent for AC, and its power flow can be controlled independently of the phase angle between the systems at either end, letting it connect grids that are not synchronized or that run at different frequencies. The technology was developed extensively in Sweden and Germany during the 1930s, and the modern era began with the Moscow-Kashira link in 1951 and the Gotland-mainland Sweden connection in 1954; current systems use solid-state thyristors and insulated-gate bipolar transistors rather than the mercury-arc valves of the earliest links. HVDC is now the technology of choice for very long transmission lines and submarine cables, exemplified by the Zhundong-South Anhui line in China, an ultra-high-voltage plus-or-minus 1,100 kilovolt connection running more than 3,000 kilometers.