A Coordination Control Method for Multi-terminal AC/DC Hybrid System Based on MMC Transmission Technology

In the voltage source converter based multi-terminal high voltage direct current system (VSC-MTDC), the power electronic topology of the modular multilevel converter (MMC) determines its characteristics of zero rotational inertia, and the fluctuation of DC transmission power may lead to short-time power imbalance in the AC system connected with the inverter, however, the response speed of thermal power unit cannot complete frequency adjustment in a short time. Regarding this kind of problem, combined with the improved droop control (IDC) method of the converter, a comprehensive control method of inverter based on virtual synchronous generator (VSG) is proposed in this paper. Firstly, on basis of analysis that the control mechanism and shortcomings of the improved droop control method for VSC-MTDC system, a VSG control method for the inverter with inertial response and primary frequency modulation is proposed. Secondly, the stability of the system is analyzed based on the second-order mathematical model of VSG. Finally, the model of five-terminal MMC-MTDC transmission system including wind farm is built based on the Opal-RT simulator. Simulations are performed to demonstrate the proposed control method by designing different operating conditions of MMC-MTDC system.