High-voltage direct current

High-voltage direct current (HVDC) is a system for transmitting electrical power using high-voltage direct current and converter stations at its terminals. It is especially useful for long-distance transmission and links between asynchronous grids.

Submarine power cable: An insulated power cable laid beneath a body of water to connect electrical networks or supply offshore loads. HVDC avoids the high charging-current limits that constrain long AC submarine cables.
High-voltage alternating current: A system that transmits electrical power over long distances using high-voltage alternating current. AC lines are often cheaper at shorter distances, while HVDC avoids some distance-dependent losses and constraints.
René Thury: A Swiss engineer who developed high-voltage direct-current transmission systems using series-connected generators and motors. His systems demonstrated practical DC transmission before modern semiconductor converters.
Grid interconnection: The physical and operational connection of electrical power systems so they can exchange electricity. HVDC provides controllable interconnections between networks that may not operate synchronously.
Asynchronous interconnection: A power link connecting electrical grids that operate at different frequencies or are not synchronized. HVDC transfers power between grids without requiring their AC waveforms to synchronize.
Back-to-back converter station: A facility containing AC-to-DC and DC-to-AC converters connected by a short DC link without a long transmission line. It uses HVDC conversion to couple grids even when no long-distance DC line is needed.
Gotland HVDC link: A 1954 submarine HVDC transmission link connecting the Swedish island of Gotland to the mainland. It was the first commercial HVDC link and introduced mercury-arc converter technology in service.
Power system stability: The ability of an electrical power system to maintain or regain a stable operating condition after disturbances. Fast HVDC power controls can support stability and limit disturbances between connected grids.
Offshore wind power: Electricity generated by wind turbines located in marine waters, typically transmitted to shore by subsea cables. Long-distance offshore projects can use HVDC to bring large power flows to shore.
Line-commutated converter: A power-electronic converter that uses the connected AC system’s voltage to switch thyristors and convert between AC and DC. Its commutation and reactive-power demands contrast with the more controllable voltage-source approach.

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