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Application of MPSO based staircase modulation strategy in MMC

A staircase modulation strategy based on modified particle swarm optimization (MPSO) algorithm is presented in this paper to solve the key problem of staircase modulation in modular multilevel converter. Firstly, each harmonic of output voltage is analyzed, and the object function is optimized. Then the modified MPSO, which has the merit of high accuracy, was adopted for calculating the switching angles. Considering the voltage balancing requirements, the square wave pulses rotation method and staircase modulation strategy are combined together to reduce the voltage ripple of floating capacitor. Voltage balancing control is achieved without extra voltage controls. Both the simulation and experimental results of five-level MMC prototype verified the feasibility and effectiveness of the proposed strategy.