Four-wired dynamic voltage restorers based on cascade open-end winding transformers

This work studies three-phase four-wire dynamic voltage restorers (DVRs) based on a cascade of open-end winding transformers. The possible proposed topologies can be observed if each transformer set is connected to the conventional two-level (2L) three-phase four leg converters and two dc-links per stage. However, in this paper particular/intermediate solutions are also considered by means of the number of fourth-legs connected at each cell and the possibility to reduce the number of dc-links without issues associated with common mode currents (CMC). The configurations are generalized for K stages. In this way, the proposed topologies can have K + 1 or 2K (the focus of this paper) dc-links connected to the converter pairs. The fourth-leg features in the converters improve the zero-sequence voltage compensation capability for the DVR and extra dc-links improves the multilevel performance. Additionally, an optimized multilevel performance is achieved by considering the transformers turns ratios. Symmetrical (i.e., equal dc-link voltages) and Asymmetrical operation (i.e., unequal dc-link voltages) cases are also considered. Modeling, PWM and control strategy are presented. Simulation and experimental results are provided to validate theoretical approaches.