Research on a cascaded multilevel inverter by employing three-phase transformers

Cascaded multilevel H-Bridge inverter is a promising topology and is an alternative for converters that are used for grid-connected photovoltaic/wind-power generators, flexible alternating current systems and motor drive applications. The cascade multilevel inverter (CMI) can flexibly expand the output power capability and favourable to develop because the converter provides modularity in topology, control structure and modulation. A recent version of CMI topology employs a single dc source and low-frequency three-phase transformers. Compared with conventional topologies, this CMI with three-phase transformers facilitates high-quality output waveforms with reduced number of components. Additionally, structure has high degree of freedom for specifying the cost function in terms of filter size, losses and total harmonic distortion (THD). In the present study, the authors proposed three major control techniques for this CMI namely (i) fundamental frequency, (ii) selective harmonic elimination PWM (SHEPWM) and (iii) sinusoidal PWM (SPWM) methods. To demonstrate presented CMI effectively, THD comparison is carried out with conventional seven-level CMI. Selected experimental results are reported to verify and validate the theoretical findings.