Theoretical and Experimental Comparison of Different Control Strategies for Modular Multilevel Converters

This paper presents an investigation of different level-shifted PWM strategies for modular multilevel converters (MMC). The control strategy of level-shifted PWM with sorting method for MMC is introduced, so the good THD performance of level-shifted PWM and simplicity of sorting method can be reached simultaneously. N+1 Modulation and 2N+1 Modulation caused by arrangement of carriers are also explained. Moreover, the harmonic content of different PWMs is analyzed and compared by analytical double Fourier integral method. The harmonic features of phase voltage, line-to-line voltage, common mode voltage, circulating current and DC link current are investigated. According to the analysis, it is found that MMC with APODPWM has a relatively clean DC link current, while better THD performance can be obtained when PDPWM is used in MMC. Finally, simulation and experimental results are provided to verify the analysis.