A Coordination Control Method for Multi-terminal High Voltage DC System Based on Modular Multilevel Converter

In modular multilevel converter-based multi-terminal DC (abbr. MMC-MTDC) transmission system the load fluctuation in AC area leads to the transitory power imbalance in this area, thus it causes the grid frequency fluctuation in this area. To cope with this problem, firstly, a virtual synchronous generator (abbr. VSG) based converter control method was proposed. Secondly, in allusion to fact that when VSG strategy was applied to frequency regulation the output power of converter station might be changed and the power balance at DC side might be broken so that too large deviation of DC voltage might occur, and such an impact would be especially evident under the existence of line impedance, for this reason, combining VSG control with improved droop control to form a comprehensive coordinated control strategy of MMC-MTDC system with multi-stations. Such a control strategy not only could provide frequency support for AC area but also could optimize the distribution of the unbalance power of DC bus to improve the stability of system DC voltage. Finally, based on Matlab/Simulink platform, a five-terminal MMC-MTDC system was constructed for the simulation. Simulation results show that the proposed control strategy is effective.