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Different power electronic converter topologies have been investigated to integrate renewable energy systems to the grid. Cascaded multilevel converters with a high-frequency link have emerged as a viable candidate for such applications. Electrical isolation can be provided using a compact high-frequency transformer connected in the link thus avoiding a bulky line frequency transformer. The use of cascaded modules allows the generation of a multilevel voltage having low total harmonic distortion but increases the overall system size. In this paper, a 15-level high-frequency ac-link single-stage asymmetrical multilevel converter for grid integration is proposed. The single-stage conversion approach eliminates the dc-link capacitors, resulting in a reduced footprint. The asymmetrical module voltages are generated by the multiwinding transformer having unequal turns on each of the secondaries. This allows the generation of 15 output voltage levels, using only three modules in each phase. A modulation strategy is proposed to generate multilevel output voltage. The effect of switch nonidealities on the output voltage is analyzed and two compensation techniques are developed to improve the voltage profile. A multiwinding high-frequency transformer is designed and characterized for the proposed converter. The presented concepts are verified by simulation and further validated experimentally on a three-phase 15-level converter prototype.
Iyer et al. (Thu,) studied this question.