Elimination of Low-Frequency Ripples and Regulation of Neutral-Point Voltage in Stacked Multicell Converters

Abstract

This paper introduces a modulation method for the stacked multicell converters (SMCs). The proposed method is implemented using phase-disposition pulse width modulation and is capable of balancing and regulating the voltages of all the capacitors in the topology, i.e., the flying capacitors and the dc-link capacitors. The proposed method is also able to eliminate the low-frequency voltage ripples that may appear in the neutral point (NP) of SMCs. In SMCs with two stacks, the NP voltage level can be generated by direct connection of the output to the dc-link NP, but also using two extra states available. This redundancy is used to regulate the NP voltage. Furthermore, since the proposed method can eliminate the low-frequency voltage ripples in the capacitors, the capacitances of all the capacitors of the SMCs can be reduced, while maintaining high quality of the output voltages and currents. This enables the use of film capacitors. Simulation and experimental results are presented from a three-phase five-level $2\times 2$ SMC to verify the effectiveness of the proposed method.