Control of a Multi-terminal DC Transmission System based on Local Variables

High voltage DC (HVDC) transmission systems have demonstrated to be a cost effective solution for transmission of electrical energy over long distances or submarine and underground cables. Among the different HVDC technologies, voltage source modular multilevel converter (VSC-MMC) provides the highest power quality, modularity, availability and, additionally, it enables the implementation of multi-terminal HVDC (MTDC). These MTDC systems require a control system which can manage the power exchange among different HVDC stations keeping the DC voltage between certain limits. Previously proposed controllers, such droop control, usually requires adaptive algorithms to tune the controller parameters. On the other hand, constant dc voltage control provide low reliability because only one station controls the voltage of the complete dc line. In this paper, the control of an MTDC system based on VSC-MMC stations is proposed. A set of power and voltage limits are defined for each station, as well as the information related to allowed operation modes. The controller in one station does not require information from other stations because its operation mode and references depend on its own local variables. Simulation results which validate the proposed control strategy are shown.