f the ratio of NPC/H and H bridge hybrid multilevel converters DC supply voltage changed, the output voltage PWM waveform will be discontinuous. Thus an improved topology and control strategy is proposed. In new method, master converter operates at the fundamental frequency based on command voltage, slave converter adopts carrier phase shifted PWM technology (CPS-SPWM) at a higher frequency to enhance the quality of output voltage waveform. The amplitude of command voltage is higher than output voltage amplitude of the master converter at any time, to ensure that output voltage variation of both converters keeps in the same direction without any current flow backwards. Matlab simulation results demonstrate the effectiveness of the proposed control strategy and topology.
