High-voltage direct current transmission (HVDC) principle is the use of stable direct current has no inductive reactance, capacitive reactance also does not play a role, no synchronization problems and other advantages of high-power long-distance direct current transmission.

The transmission process is DC. Commonly used in submarine cable transmission, non-synchronous operation of the AC system between the connection and so on.

High-voltage DC transmission technology is used to transmit power over long distances via overhead lines and submarine cables; at the same time, it is also used to connect independent power systems in situations where traditional AC connections are not appropriate. The world's first commercialized HVDC transmission line was built in Sweden in 1954 to connect the Swedish mainland to Gotland.