High-voltage Direct Current
A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance each cycle. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be warranted, due to other benefits of direct current links.
HVDC allows power transmission between unsynchronized AC distribution systems, and can increase system stability by preventing cascading failures due to phase instability from propagating from one part of a wider power transmission grid to another. HVDC also allows transfer of power between grid systems running at different frequences, such as 50 Hz vs. 60 Hz. Such interconnections improve the stability of each grid, since they increase the opportunity for any grid experiencing unusual loads, to stay in service by drawing extra power from otherwise completely incompatible grids.
The modern form of HVDC transmission uses technology developed extensively in the 1930s in Sweden (ASEA) and in Germany. Early commercial installations included one in the Soviet Union in 1951 between Moscow and Kashira, and a 100 kV, 20 MW system between Gotland and mainland Sweden in 1954. The longest HVDC link in the world is currently the Xiangjiaba–Shanghai 2,071 km (1,287 mi), ±800 kV, 6400 MW link connecting the Xiangjiaba Dam to Shanghai, in the People's Republic of China. In 2012, the longest HVDC link will be the Rio Madeira link in Brazil, which consists of two bipoles of ±600 kV, 3150 MW each, connecting Porto Velho in the state of Rondônia to the São Paulo area, where the length of the DC line is over 2,500 km (1,600 mi).
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