Submodule Capacitor Voltage Regulation of Modular Multilevel Converter Based Traction Propulsion System With Medium Frequency Isolation

This article presents a traction converter configuration using modular multilevel converter (MMC) for interfacing the single-phase ac railway power line with the multiple traction motors of an electric locomotive. The conventional grid frequency step-down transformer used in electric locomotive is replaced by a medium frequency transformer (MFT). The system consists of two sets of MMCs: one to interface the power line with the MFT, and another to interface the MFT with the traction motor. The overall control philosophy of the system is presented. Furthermore, a control technique is proposed that reduces the peak to peak ripple voltage of the submodule capacitors of the motor side MMC. The system is simulated in MATLAB Simulink. The proposed control technique for the motor side MMC is experimentally verified in a laboratory prototype.