Improving power quality in cascade multilevel converters based on single-phase non-regenerative power cells

The existence of inter-harmonics in the line currents demanded by AC/DC/AC converters is mainly due to the light decoupled behavior of the DC link filters; particularly, when different input/output operating frequencies are used. Current sub- and inter-harmonics affect the operation of power transformers, causing a reduction in the system efficiency and power quality. Inter-harmonics can be very evident in converters with single-phase stages, as they require large capacitors to filter out the voltage and current second harmonic present in the DC links, which usually is not fully mitigated. This is the case of the cascade multilevel converter based on single-phase power cells, where each power cell has a single-phase rectifier and a single-phase inverter that become not totally decoupled with standard size capacitors. This paper shows that it is possible to improve the quality of the input currents of such power cells even when the input/output frequencies are different. This is achieved by means of magnetic couplings among the DC links of the power cells that feed different output phases.