Cascaded Transformer Symmetric Single-Phase Multilevel Converters With Two DC Sources

Abstract

This article investigates three families of symmetric single-phase multilevel converters with two dc links for inverter applications. The proposed configurations are based on cascaded cells formed by two-level legs and injection transformers that are connected to a basic cell formed by two half H-bridges with shared-legs. Three types of cells are proposed: cell <inline-formula><tex-math notation= LaTeX >$U$</tex-math></inline-formula>, composed of three legs and two transformers; cell <inline-formula><tex-math notation= LaTeX >$V$</tex-math></inline-formula>, composed of four legs and three transformers; and cell <inline-formula><tex-math notation= LaTeX >$W$</tex-math></inline-formula>, composed of six legs and four transformers. The topologies operate in the nonisolated condition (with one less transformer). The generalized system model equations of proposed topologies are discussed. The transformer turns ratios and the dc-link voltages are determined in order to improve the system symmetry and generate the output voltage with low harmonic contents. An unidimensional pulsewidth modulation technique is presented to control the switches of the converters. Harmonic distortion analysis, number of components, rating of the semiconductor devices, power semiconductor losses, IGBT and diode temperature analysis, and transformer ratings are used to compare the proposed configurations with a modular symmetric multilevel inverter formed by three-leg modules cascaded thought injection transformers. The proposed converters present advantages as higher quality of output voltages, and lower semiconductor power losses using the same or lower number of circuit components. Experimental results are shown to demonstrate the feasibility of the systems.