Balanced Submodule Operation of Modular Multilevel Converter-Based Induction Motor Drive for Wide-Speed Range

Operation of the conventional modular multilevel converter (MMC) at low frequencies for the induction motor drive is difficult. The peak-to-peak voltage ripple of submodule (SM) capacitor increases abnormally at low frequencies of the drive. Recently, the problem of high voltage ripple has been solved by using back-to-back MMC without overloading the converter till very low frequency of the drive. In this back-to-back configuration, the grid-side MMC generates constant dc current source at its output instead of dc voltage source. Using this current source as input, the motor-side MMC drives a three-phase induction motor. It is shown that the voltage ripple of SM capacitor remains constant till very low frequency due to the dc current source. However, the average capacitor voltage controllers do not guarantee balanced operation of individual SM. In this article, the need for balancing controller of individual SM capacitors of back-to-back MMC is established experimentally. Therefore, this article proposes additional voltage-balancing controllers for the individual capacitor of grid-side MMC and motor-side MMC without disturbing the average controllers. The operating principle of this balancing controller has been presented analytically and verified experimentally. Finally, the operation of the drive with the proposed balancing controller is presented for wide speed range.