Improving Power Quality in Cascade Multilevel Converters Based on Single-Phase Nonregenerative Power Cells

Interharmonics exist in the ac supply currents in ac/dc/ac drives mainly due to the poor decoupled behavior of the dc-link stage. This issue is particularly evident when different input/output operating frequencies are used, causing harmful effects on power transformers and reducing the system efficiency and power quality. Consequently, large interharmonics can be found in converters with single-phase stages, as they require large electrolytic capacitors to filter out the dc-link second harmonic of voltage and current, which is usually not fully accomplished. This is the case of the cascade multilevel converter based on single-phase power cells, where each module has a single-phase rectifier and a single-phase inverter stage that cannot be effectively decoupled with standard size capacitors. This paper shows that it is possible to improve the quality of the power cell input currents when the input/output frequencies are different in the cascade multilevel converter. This is achieved by means of magnetic couplings among the dc-links of the power cells that feed different output phases, while keeping the high power quality on the load side.