Modular Multilevel Converter Based Variable Speed Drive With Reduced Capacitor Ripple Voltage

In recent years, modular multilevel converter (MMC) is considered for medium-voltage variable speed drives (VSD) due to its modularity and higher reliability. However, the major drawback of the MMC-based VSD is the higher capacitor ripple voltage at lower operating speeds. In literature, this ripple voltage is reduced by injecting a high-frequency circulating current into each arm. This additional circulating current increases the overall current rating of the devices, inductors, and capacitors. Alternatively, the capacitor ripple voltage can be reduced at the lower output frequency by reducing the dc-bus voltage. This paper proposes a new configuration using a multipulse diode-bridge rectifier to reduce the dc-bus voltage when the drive is operated at lower speeds. Therefore, the capacitor ripple voltage decreases without injecting any circulating current. In addition, a new technique is proposed to maintain the peak-to-peak value of the submodule capacitor ripple voltage constant at near-zero speed. The proposed converter with 24-pulse rectifier configuration is simulated in MATLAB/Simulink, and the experimental validation is carried out on a prototype with 12-pulse rectifier.