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

High voltage direct current (HVDC) systems, with its advantages of large capacity, long distance, high efficiency, and low loss, are becoming the core support of the new power system. However, in conventional droop control, the fixed droop coefficient causes output power disproportionate to their available capacities among converters, as well as 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, minimization of voltage deviation and maximization of capacity utilization rates of converters. Further, a cooperative control based on optimization model is proposed, where information of 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 make power mismatches proportional to their available capacities among converters. Finally, a testbed is built in PSCAD/EMTDC and three 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 of out of service of a converter and the three-phase ground fault of a converter, respectively. 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 increased by 24.46% and 18.75%, respectively.