Analysis of Harmonic Characteristics of Asymmetric Bridge Arm Parameters in MMC

Modular multilevel converter (MMC) have many advantages over traditional voltage source converters. It is used more and more widely. However, it is inevitable to run in an unsatisfactory situation actually. The asymmetry of the bridge arm inductance parameter causes imbalance in the operating conditions, which causes harmonic problems on the AC side and the DC side in turn. In this paper, the average switching function theory is used to establish the harmonic mathematical model of bridge arm voltage and phase-to-phase circulation for analyzing the phase voltage and DC current of the AC port. It shows that asymmetry of the single-phase bridge arm inductance will cause the asymmetrical odd-order harmonic in the circulation, and asymmetry of phase bridge arm parameters causes asymmetrical even harmonic. The harmonic amplitude is positively related to the asymmetry. The asymmetry of bridge arm introduces DC component and even harmonics such as second harmonic into the AC port; The fluctuation on the DC side is mainly based on fundamental frequency, double frequency and triple frequency. The simulation results verify the correctness of the proposed mathematical model and the law.