Harmonic Analysis of Pulse Width Modulation-Based Strategies for Modular Multilevel Converter

MMC converters are an interesting solution for HVDC applications and renewable energy transport. This paper presents a mathematical analysis of the impact of various Phase Disposition PWM strategies on the circulating current which is one of the important MMC converter issues. The considered strategies are: Phase Disposition PWM (PDPWM), Phase Opposition Disposition PWM (PODPWM) and Alternative Phase Opposition Disposition PWM (APODPWM). Depending on the adapted modulation strategy, the proposed analysis proves that the circulating current varies from a DC component to a high current with high frequency components and a dominance of the second order harmonics. It is also demonstrated that PODPWM and APODPWM modulation strategies satisfy the balanced operation condition where only N sub-modules among the 2N available sub-modules are inserted into the phase leg and consequently ensure (N+1) voltage levels in the output voltage. Simulation results of a 2.7 MVA MMC converter are presented to verify the validity of the proposed methodology and analysis.