Interaction Between AC Grids and MTDC Systems Based on Droop Controllers

HVDC multi-terminal systems enhance operational flexibility of the power transmission systems as result of the control of different HVDC stations. This paper firstly describes the distributed DC voltage control based on the voltage droop technique combined with the DC voltage control given by a single VSC. Furthermore, the frequency droop control is attached in some VSCs to give support in the frequency regulation of the AC grid. Both controllers are theoretically developed, and their interaction exposed. A five terminal MTDC grid is simulated to show the interaction between the AC grids and the HVDC multi-terminal system under two main study cases: isolated and interconnected AC grids. Finally, an analogy between DC voltage control of the MTDC system and the frequency control of the AC grids is depicted.