Reduction of capacitor ripple voltage and current in Modular Multilevel Converter based variable speed drives

The major drawback of Modular Multilevel Converter (MMC) based variable speed drives is that the capacitor voltage ripple varies inversely with the output frequency. This voltage ripple can be reduced at lower operating frequency by injecting circulating current into its each arm. However, this circulating current increases the overall current rating of the converter. The voltage ripple can also be reduced by reducing the dc bus voltage when the drive is required to be operated at lower speeds. In this paper, a new configuration using multipulse diode bridge rectifier is proposed to reduce the dc bus voltage when the drive is operated at lower speeds. Therefore, the capacitor ripple voltage is reduced without injecting any circulating current. The proposed configuration is simulated in MATLAB/Simulink and the simulation results are presented to validate the proposed configuration.