Fast Voltage-Balancing Control and Fast Numerical Simulation Model for the Modular Multilevel Converter

A methodology of fast voltage-balancing control based on average comparison and a fast numerical simulation model for the modular multilevel converter are proposed. In each control cycle, the average voltage of all the submodules (SMs) in an arm is calculated. The switching state of each SM will be determined by comparing the capacitor voltage of each SM with this average value. Little sorting of the capacitor voltages is required; therefore, the calculation burden on the controller is significantly reduced. The previous switching state of each SM will be mostly retained. Two variables △U_{C_sort} and △U_{C_max} are introduced to adjust the equivalent switching frequency of each SM. Simulation results verified the effectiveness of the proposed voltage-balancing control in normal and fault conditions. A fast numerical simulation model for MMC on PSCAD/EMTDC is also proposed. The dynamics of each SM are sufficiently represented in the proposed model. A hybrid simulation model is further developed to incorporate the detailed switching dynamics of specific interested SMs. The simulation results show that the proposed models give almost identical results as the detailed switching models while the simulation speed of the proposed model is approximately 5000 times faster than the detailed switching model.