Medium-Voltage Vector Control Induction Motor Drive at Zero Frequency Using Modular Multilevel Converter

Conventional modular multilevel converter (MMC) has severe limitation to operate at low frequency for the application of variable-speed induction motor drive. Fluctuation of submodule (SM) capacitor voltage increases rapidly as the motor speed decreases with rated torque. Finally, it becomes impractical to operate the motor at zero frequency. Recently, MMC-based medium voltage induction motor drive has been proposed to tackle this problem without additional circulating current. However, it does not solve the problem near zero frequency with nonzero motor torque. This paper proposes new method to solve this problem of high peak-to-peak voltage ripple of SM capacitor at all frequencies including zero frequency. Vector control strategy for induction motor is adopted here to have accurate control of torque near zero frequency. First, the problem of low-frequency operation is analyzed. Then, the solution is proposed analytically. The proposed control technique and overall performance of the drive have been verified through experimental results.