A high voltage direct current (HVDC) electric power transmission system uses direct current for the massive transmission of electrical energy, in contrast to the most common AC systems. . For long-distance transmission, HVDC systems can be less expensive and suffer less electrical losses. For submerged power cables, HVDC avoids the intense currents required to charge and discharge cable capacity in 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 non-synchronized AC transmission systems. Since the power flow through an HVDC link can be controlled independently of the phase angle between the source and the load, it can stabilize a network against disturbances due to rapid power changes. HVDC also allows the transfer of energy between network systems operating at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid, as it allows the exchange of energy between incompatible networks.
The modern form of HVDC transmission uses technology developed extensively in the 1930s in Sweden (ASEA) and in Germany. The first commercial facilities included one in the Soviet Union in 1951 between Moscow and Kashira, and a 100 kV and 20 MW system between Gotland and mainland Sweden in 1954. The longest HVDC link in the world is 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 with the São Paulo area. The length of the DC line is 2,375 km (1,476 mi).
In July 2016, the ABB Group was awarded a contract in China to build a UHVDC with a voltage of 1100 kV, a length of 3,000 km (1,900 mi) and 12 GW of power, setting world records for the highest voltage, the longest distance and the highest transmission capacity.