Sensorless vector control scheme for an induction motor fed by MMC

Due to its notable advantages, Modular Multilevel Converter (MMC) is considered to be employed for medium-voltage motor drives. In this paper, a high-performance sensorless vector control scheme inspired from the Model Reference Adaptive System (MRAS) approach is developed for an MMC-fed induction motor. Besides the well-known vector control scheme advantage, i.e. independent control of motor flux and torque, eliminating the speed sensor will notably improve the reliability of the proposed method. The motor flux components in the alpha beta frame are determined and used for estimating the speed value. As the errors of flux components calculated by the motor voltage and current models are continuously compensated by a MRAS-based loop, the estimation would be accurate for a wide range of speeds. Moreover, several modifications are considered to add the capability of voltage balancing for submodule capacitors, and also circulating current control to the proposed scheme. The two aforementioned features are necessary for efficient operation of MMC. Moreover, the circulating current control capability is mandatory for injection of different circulating current profiles, which is required for reducing the flying capacitors' voltage ripples at low speeds. Finally, different performance aspects of the proposed method are experimentally assessed and validated.