Abstract
Submodule (SM) capacitors used in Modular Multilevel Converters (MMCs) often tend to be bulky and costly. For specific operating conditions recent studies have demonstrated operation of MMCs under reduced SM capacitance. This paper brings a detailed and systematic analysis of low-capacitance MMC operating in grid-connected mode, and with the circulating currents actively controlled to zero. Firstly, it reveals that the increased capacitor voltage ripple affects the modulation references of SMs, which under low SM capacitance include a significant 2nd harmonic component and a significant dc-shift component which cannot be neglected and have to be taken into account for accurate design. The 2nd harmonic modulation reference is the result of closed loop circulating current control and the dc-shift in modulation is the result of average SM capacitor voltage control. Secondly, it analytically models complex interactions among various harmonic components of SM capacitor voltage ripple and the associated modulation reference i.e., ac-fundamental, 2nd harmonic and dc-shift in modulation, and it demonstrates the effects of these interactions on both ac and dc components of SM output voltages. The analytical model serves as means to analyze and design low capacitance MMCs. The presented analysis is supported by simulation.
Published Version
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