A broad range of frequency control for the modular multilevel cascade converter based on triple-star bridge-cells (MMCC-TSBC)

This paper focuses on an application of the modular multilevel cascade converter based on triple-star bridge-cells (MMCC-TSBC) to adjustable-speed motor drives. The TSBC is one of direct ac-to-ac power converters capable of bidirectional power flow with three-phase sinusoidal input and output currents. Therefore, it is suitable to medium-voltage high-power motor drives requiring regenerative braking. This paper designs, constructs, and tests a three-phase 400-V, 15-kW, downscaled model with four bridge-cells per cluster, adopting the so-called "phase-shifted PWM" to produce multilevel waveforms at both sides. Good operating performance is verified by some experimental results obtained from the downscaled model. On the other hand, the TSBC suffers from such a serious problem that capacitor-voltage fluctuation becomes larger as the motor (output) frequency is closer to the supply (input) frequency. This paper also has an intensive discussion on how to control the TSBC intended for achieving a broad range of frequency control. Theoretical analysis and computer simulation confirm the effectiveness and viability of the proposed control method.