Interaction of Droop Control Structures and Its Inherent Effect on the Power Transfer Limits in Multiterminal VSC-HVDC

Future multiterminal HVDC systems are expected to utilize dc voltage droop controllers, and several control structures have been proposed in the literature. This paper proposes a methodology to analyze the impact of various types of droop control structures using small-signal stability analysis considering all possible combinations of droop gains. The different control structures are evaluated by the active power transfer capability as a function of the droop gains, considering various possible stability margins. This reveals the flexibility and robustness against active power flow variations, due to disturbances for all of the implementations. A case study analyzing a three-terminal HVDC VSC-based grid with eight different kinds of droop control schemes points out that three control structures outperform the remaining ones. In addition, a multivendor case is considered where the most beneficial combinations of control structures have been combined in order to find the best performing combination.