Analysis and Control of Grid-Tied Modular Multilevel Converters with a Passive Front-End Rectifier without LC Filter in the DC-Link

This paper determines new design and control requirements of a grid-tied full-bridge Modular Multilevel Converter (MMC) connected to a diode-bridge front-end without passive components in the dc-link for wind energy conversion systems. It develops a new steady-state model of the full-bridge MMC with non-ideal dc-link voltage where MMC input impedance (
$Z_{MMC}$
) is modelled as a function of MMC parameters including the dc-link dominant harmonic frequency. The analysis reveals that the ripple in the dc-link can create a new resonance frequency of the MMC sub-modules and arm inductances, in addition to the conventional second-order resonance frequency, which however can be prevented by optimal selection of MMC parameters to avoid conditions at which
$Z_{MMC}$
equals to zero. Furthermore, the paper proposes a new MMC control structure to actively suppress current harmonics in the dc-link and MMC arms leading to elimination of input-filter requirements in the proposed topology. The developed average model and control are verified through simulation tests.