Droop-based Control Scheme for MMC-HVDC Connected to Weak / Very Weak Grids

This paper proposes a droop-based control scheme for Modular multilevel converter (MMC) based HVDC systems connected to weak / very weak grids. The control scheme mainly includes P-f and Q-V droops, d-q voltage closed-loop, d-q current closed-loop, circulating current suppression, as well as submodule (SM) voltage balancing controls. Using droop control, frequency is obtained by controlling active power flow and Phase Locked Loop (PLL) which is usually used to obtain the frequency is avoided. Thus, MMC-HVDC systems with the proposed control scheme overcome the obstacles caused by PLL in weak grids applications. A 100 kW grid-connected MMC-HVDC system is built and simulated in Matlab/Simulink, and two control schemes, traditional vector control and droop-based control, are used to control the system. Performance of the MMC-HVDC system with two different control schemes are analyzed and compared. Comparison results show that vector control fails under weak / very weak ac grid condition, while the proposed droop-based control works well under both strong and weak grid conditions. Simulation results demonstrate the effectiveness of the proposed control scheme applied for MMC-HVDC connected to weak / very weak grids.