A high voltage, direct current (HVDC) power transmission system (also called a super-highway or an electric highway) uses direct current for the transmission of bulk electrical energy, in contrast to the most common current alternatively. For long-distance transmission, HVDC systems can be less costly and suffer lower electrical losses. For submarine 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 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 power between network systems operating at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid by allowing the exchange of energy between incompatible networks.
The modern form of transmission HVDC uses technology developed extensively in the 1930s in Sweden (ASEA) and 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, in two bipolar units of ± 600 kV, each of 3150 MW, connecting Porto Velho in the state of Rondônia to the São Paulo area. The length of the DC line is 2,375 km (1,476 mi).
In July 2016, ABB Group received a contract in China to build a UHVDC link with a voltage of 1100 kV, a length of 3,000 km (1,900 mi) and 12 GW of power, establishing world records for the highest voltage, the longest distance and the largest transmission capacity.