Asymmetrical Cascaded Three-Phase AC-DC Converters With Injection Transformers

In this paper, three generalized configurations of three-phase pulsewidth modulated (PWM) rectifiers are presented. They are composed of cascaded cells, each formed by a three-phase bridge of switching devices and a three-phase transformer to cascade the cells and isolate the load from the power source. One configuration is bidirectional, whereas the other two are unidirectional alternatives that replace power switches for diodes in some legs. The turns ratios of transformers from each cell are designed in order to provide the required gain and number of levels in the inverted multilevel voltage, enhancing power quality while attending requirements for proper operation in each case. Configurations with two and three cells are compared by means of simulations between themselves and with three conventional three-phase PWM rectifiers: the two-level insulated gate bipolar transistor bridge and the multilevel neutral point clamped (NPC) bridge and NPC H-bridge. Many degrees of asymmetry between the transformers in each cell, determined by the turns ratios, are considered to evaluate their impact on the topologies with relation to performance and cost. It is shown that proposed multilevel rectifiers present lower harmonic distortion, semiconductor losses, and switching stress, and some options with intermediate asymmetry are considered as good alternatives to the most asymmetrical cases. Experimental results are also provided for configurations with two cells, to demonstrate their lower harmonic distortion and power losses compared to the conventional isolated two-level PWM rectifier.