The use of direct current (dc) power networks,
either at high voltage or at medium voltage, is being
increasingly seen in modern smart grids. This is due to the
flexible control possible with dc and its ability to transmit
and distribute power under circumstances where alternating
current (ac) networks are either unable to, or less economic.
This paper provides an overview of the evolution of high voltage dc (HVDC) transmission from early Thury systems, to modern ultrahigh-voltage dc and multiterminal voltage source converter systems. The operation of both current source and voltage source systems is discussed, along with modeling requirements. The paper provides a snapshot of the state of the art of HVDC with copious references to enable in-depth reading. Key developments over the last 20 years are highlighted. Issues surrounding multiterminal operation and dc protection are explained, along with drivers in economics and policy. This will
inform HVDC integration into smart grids.