High-voltage direct current
A high-voltage direct current (HVDC) system uses direct current (DC) and high voltages (currently between 100 kV and 800 kV) for electric power transmission. It contrasts with the more common alternating current (AC) transmission systems.
HVDC lines are commonly used for long-distance power transmission, since they require fewer conductors and incur less power loss than equivalent AC lines. HVDC also allows power transmission between AC transmission systems that are not synchronized. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. HVDC also allows the transfer of power between grid systems running at different frequencies, such as 50 and 60 Hz. This improves the stability and economy of each grid, by allowing the exchange of power between previously incompatible networks.
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 the Zhundong–South Anhui link in China, a ±1,100 kV, ultra HVDC line with a length of more than 3,000 km (1,900 mi).
