Cooperative Control Method Based on Two-Objective Co-Optimization for MMCs in HVDC Systems

High-voltage direct current (HVDC) systems, with their advantages of large capacity, long distance, high efficiency, and low loss, are becoming the core support of new power systems. However, in conventional droop control, the fixed droop coefficient causes output power disproportionate to the available capacities among converters, as well as a relatively large deviation of DC voltage in HVDC systems. Therefore, in this paper, a two-objective optimization model for droop control is developed and then it is integrated to a cooperative control, which achieves the co-optimization of voltage deviation and power sharing among multiple converters. In the optimization model, there are two objectives, the minimization of voltage deviation and maximization of the capacity utilization rates of converters. Further, a cooperative control method based on the optimization model is proposed, where information on voltage and power in droop-controlled converters is acquired and the co-optimization of voltage deviation and power sharing is performed to obtain the optimal droop coefficients for these converters, which minimizes voltage deviation, and at the same time, makes power mismatches proportional to their available capacities among converters. Finally, a testbed is built in PSCAD/EMTDC and four cases are designed to verify the proposed method under different settings. The simulation results show that compared with conventional droop control, the voltage deviation is reduced by 71.74% and 67.67% under the cases that a converter is out of service and the three-phase ground fault of a converter occurs. Additionally, when large power fluctuations occur twice, the power mismatches are shared proportionally to their available capacities, which results in the capacity utilization rates of the droop-controlled converters increasing by 24.46% and 18.75%, respectively.