This is a list of notable high-voltage direct-current power transmission projects. HVDC projects for long-distance hvdc och ac pris have two or rarely, more converter stations and a transmission line interconnecting them. Generally overhead lines are used for interconnectionbut an important class of HVDC projects use submarine power cables. A back-to-back station has no transmission line and connects two AC grids at different frequencies or phase counts. Historical HVDC systems used the Thury system of motor-generators but these have all been made obsolete by later developments such as mercury-arc valves now also obsoletethyristors, and IGBT power transistors.
HVDC is used as an alternative to AC for transmitting electrical energy over long distances or between AC power systems of different frequencies. Some HVDC systems take full advantage of this bi-directional property for example, those designed for cross-border power trading, such Frågelådan the Cross-Channel link between England and France. In such schemes, power flow in the non-preferred direction may have a reduced capacity or poorer efficiency. HVDC converters can take several different forms. Early HVDC systems, built until the s, were effectively rotary converters and used electromechanical conversion with motor - generator sets connected in series on the DC side and in parallel on the AC side. However, all HVDC systems built since the s have used electronic static Frågelådan. Electronic converters for HVDC are divided into two main categories.