Reviews important aspects of high-voltage direct-current (HVDC) power transmission systems. They summarize the history of HVDC transmission and discuss the economic and technical factors responsible for the development of HVDC systems. Terminal design and basic configurations of HVDC systems as well as major equipment of HVDC transmission systems are considered, along with the state-of-the-art technology in equipment construction. Future developments in HVDC transmission systems are dealt with, including promising technologies such as multiterminal configurations, gate-turn-off (GTO) devices, forced commutation converters, and advanced control electronics. >