Performance evaluation and selection of PWM switching and control methods for grid connected modular multilevel converters

This paper focuses on the determination of suitable carrier based pulse width modulation (PWM) switching and control methods for grid connected modular multilevel converters (MMCs). Characterization of various level-shifted and phase-shifted carrier based PWM methods are provided in terms of output voltage waveforms both for N+1 and 2N+1 level output phase voltages. Carrier based PWM method based control approaches are evaluated for MMC. Performances of different control methods are evaluated and compared for output voltage harmonics distortion, submodule capacitor voltage ripple, circulating current ac component, and energy efficiency under equal switching count per phase leg criterion. The study is conducted for the dc/ac conversion stage of a 10 MW medium-voltage grid-connected back-to-back MMC system. It is found that direct modulation sort and select method with reduced switching count with 2N+1 level output voltage stands out with its performance.