New Island Robust Control for Renewable Energy Generation Sent Through Islanded MMC-HVDC

The renewable energy generation sent through the islanded MMC-HVDC system is susceptible to the output changes and the faults in AC or DC. When a traditional island control is disturbed, the stable operating point shifts, which affects the stability of the output AC voltage in the sending-end converter station, even severely causing a crash in the system. Therefore, this paper proposes a new type of island robust control: Firstly, a second-order mathematical model of the sending-end converter station is established, a third-order linear extended state observer (LESO) is designed to estimate the disturbance of the system in real time and be feedforward compensated to improve the ability of anti-disturbance, and the proportional-integral (PI) controller is used to control the stability of the grid-connected point voltage. Secondly, the convergence and stability of the proposed control system are analyzed. Finally, the link of the fault current suppression is designed to reduce the overcurrent risk of the MMC after a fault. Adopting a compound control of feedforward with feedback, the new island control simplifies the controller structure while improving its robustness. The simulation model of renewable energy generation sent through a four-terminal islanded MMC-HVDC system is built in the PSCAD/EMTDC to verify the effectiveness of the control strategy proposed in this paper. The simulation results show that the new island control has a superior transient response and recovery response under the disturbance of the output changes of the renewable energy and the faults in AC or DC.