A Mode-switching Strategy of Droop Control for VSC-MTDC Systems Considering Maximum DC Voltage Regulation Capability

To achieve the goal of carbon neutrality, renewable energy integration through a voltage source converter based multi-terminal direct current (VSC-MTDC) system has been identified as a promising solution. To tackle the significant DC voltage over-limit problem in a VSC-MTDC system during disturbances, this paper proposes a mode-switching strategy of droop control considering maximum DC voltage regulation capability. The close relationship between node injection powers and node DC voltages in the MTDC system is elaborated, and the most effective regulation approach of local injection power for limiting DC voltage deviation is presented. The operating point trajectories of different droop control explains that the DC voltage deviation can be minimized by fully utilizing the capacity of converters. Therefore, the mode-switching strategy with the maximum DC voltage regulation capability is realized by the switching between the voltage droop control and the constant maximum power control. In addition, a mode recovery process and a smooth switching method are developed to make converters regain the capability of maintaining DC voltage and reduce power fluctuation during mode switching, respectively. Furthermore, three cases are investigated to verify the effectiveness of the proposed mode-switching strategy. Compared with simulation results of the conventional droop control and the DC voltage deviation-dependent droop control, better performance of transient and steady-state DC voltage deviation is achieved through the proposed strategy.