A Modular Quasi-Multilevel Converter using 10kV SiC MOSFETs for Medium-Voltage Cable-Fed Variable-Speed Motor Drives

This paper presents a modular multilevel converter (MMC) topology, based on high-voltage SiC power devices, for medium-voltage variable-speed drive applications that require connection cables between the inverter cabinet and the motor unit. Instead of using standard multilevel modulation, the presented MMC topology is modulated using a quasi-multilevel scheme which generates a two-level-like PWM voltage waveform, however with a controlled voltage slew-rate (dv/dt) similar to that of a standard multilevel waveform. The time setting of the quasi-multilevel modulation scheme is optimally controlled to mitigate the overvoltage oscillations that usually appear at the motor terminals due to the reflected voltage phenomenon. The quasi-multilevel modulation also enables the MMC to efficiently operate with reduced cell capacitance and diminished arm inductance requirement, regardless of the operating frequency, which reduces the drive system size and complexity. A further topology simplification is achieved by using high-voltage (10kV) SiC power devices that allow generating low-order multilevel voltage waveforms while fulfilling the medium-voltage level blocking requirements. The structure and operation of the presented quasi-multilevel converter are illustrated in this paper and its performance is assessed using a simulation study.