DC Grid Control Concept for Expandable Multi-terminal HVDC Transmission Systems

In areas where there are already several HVDC links in operation or under construction, it is more likely that a DC grid emerges as a gradual process. Although the DC grid control concept has been widely discussed, the implementation in the expandable multi-terminal HVDC (MTDC) transmission systems still faces challenges. The existing converter control system needs to be adjusted when a point-to-point (PtP) link is operated into MTDC system, or when an additional converter is interconnected with an existing MTDC system, or even when a new DC grid control strategy is applied. In this paper, a new DC grid control concept is proposed that conserves and uses the existing converter control when an existing PtP link is extended into MTDC system or the existing MTDC system is expanded. An interface is proposed to decouple the DC grid secondary with converter control structures. By using this interface, three different control modes, i.e. DC voltage, active power/current flow, and DC voltage droop control, can be achieved by simply changing the droop line characteristics of the converter. Furthermore, this interface also provides the possibility to change the response time of the existing DC voltage control and the use of auxiliary control functions, e.g.: frequency, power oscillation damping, and emergency power controls. Simulation results show that the converter stations can be coordinated using the proposed DC grid control concept. It is expected that the proposed DC grid control concept can pave the way to realize multi-vendor MTDC transmission systems.