A Modulation Strategy to Operate Multilevel Multiphase Diode-Clamped and Active-Clamped DC–AC Converters at Low Frequency Modulation Indices With DC-Link Capacitor Voltage Balance

This paper proposes a modulation strategy for multilevel multiphase diode-clamped dc–ac converters (also applicable to other functionally equivalent topologies) able to keep the dc-link capacitor voltages balanced under passive front ends, low frequency modulation indices (i.e., low number of switching transitions per fundamental cycle), any value of the amplitude modulation index, and any ac-side displacement power factor. A suitable phase voltage pattern with minimum number of switching transitions is presented for the n-level three-phase case. Subsequently, it is extended to higher number of switching transitions per fundamental cycle and to higher number of phases. Simulation results with three, four, and five levels; three and five phases; and several frequency-modulation-index values are presented to validate the proposed modulation strategy. Experimental results obtained with a four-level three-phase dc–ac converter prototype are also provided. The proposed modulation strategy enables the use of this type of converters in applications where the ac fundamental frequency may be close to the switching frequency, such as in high-power systems and variable-speed motor drives.