Modular Multilevel Converter based Variable Speed Drives with Constant Capacitor Ripple Voltage for Wide Speed Range

Modular multilevel converter (MMC) is one of the recent developed converter for high power applications. However, the major drawback of this converter in variable speed drives is that the capacitor ripple voltage increases at lower speed, when the load torque remains constant. This ripple voltage can be reduced by reducing the dc-bus voltage at lower speeds, while the capacitor voltages are maintained at their rated value. In this paper, a full-bridge submodule based MMC is proposed as front end rectifier in back-to-back MMC based drives to reduce the dc-bus voltage when the drive is operated at lower speeds. Therefore, the capacitor ripple voltage is maintained constant over the wide speed range of operation for constant load torque. This also enables the drives with regeneration capability and eliminates the requirement of complex phase-shifting transformer at the input side. The proposed configuration is simulated in MATLAB/Simulink and the simulation results are presented.