This paper presents a framework of a multi-terminal HVDC transmission system and its multi-functional control strategy. The framework possesses the basic characteristics of the DC-grid and is suitable in integrating distributed power sources. The paper proposes the first architecture for a multiterminal HVDC transmission system using the VSC technology. Its control strategy offers various functionalities that include controls for operation mode

start-up and shutdown

DC voltage

and station online re-connecting

which are significantly different from the control of point-to-point VSC-HVDC systems. The framework has not only been evaluated in real-time simulation studies

but has also been implemented onsite for the first time via the China Southern Grid's Nan'ao Multi-terminal VSC-HVDC (VSC-MTDC) project. This paper gives a brief review of the current research and engineering achievements in this field

which includes four aspects: the architecture of the VSC-MTDC system

the structure of the control and protection system

simulation verification tests setting

and the results of real-time hardware in hardware in loop (HIL) simulation studies and onsite tests.