Although many high-power voltage source converters are of three-phase three-wire type, there may also be other types of configuration with which tripplen harmonics in the ac voltage and/or current are of great concern. This paper analyzes the harmonic characteristic of the output voltage of modular multilevel converter (MMC) in cases where the third-order harmonic voltage and current are inevitable, such as in three-phase four-wire configurations. The interaction among the capacitor voltage, arm current, and the modulating signal will generate multiple harmonics in the output voltage, in which the third-order harmonic is the most dominant. The third-order harmonic in the MMC output voltage will cause excessive amount of third-order harmonic in the output current in grid-connected applications with three-phase four-wire configuration. Analytical expression for the output voltage is derived for open-loop case, considering both the fundamental-frequency reference signal and the circulating current-control signal in the modulating signals. A feedforward compensation (FFC) method is proposed for suppressing the influence of the third-order harmonic in the output voltage for grid-connected applications. Simulation and experimental results verified the correctness of the analysis and effectiveness of the proposed FFC method.
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Wu et al. (2015) studied this question.
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