Cascaded Open-End Winding Transformer Based DVR

This paper introduces and generalizes a class of multilevel dynamic voltage restorer (DVR) for voltage sags/swells compensation of high-power sensitive loads. Such a device can improve the power quality of sensitive loads located in stiff systems. The proposed DVR is based on three-phase bridge converters series-connected by means of cascaded transformers using the concept of open-end winding (OEW). Hence, two dc-links can provide either symmetrical (i.e., equal dc-link voltages) or asymmetrical (i.e., different dc-link voltages) operation of the DVR converters. A generalization for K-stages is presented as well. The proposed configuration is named DVR-cascaded open-end winding (COEW). Such a topology permits a generation of a maximized number of voltage levels per converter leg. An alternative solution with multiple dc-links (i.e., multiple dc sources), named DVR-MSCOEW, is briefly introduced. The multilevel waveforms at the output voltages of the converter are generated by using a suitable pulse-width modulation (PWM) strategy associated with both dc-link voltage and transformers turns ratios. In the scenario, where common mode currents (CMC) for DVR-COEW are introduced, and if such currents are significant, a modified PWM strategy is described to compensate for CMC issues. Since it uses two-level cells, the modularity feature makes the proposed DVR-COEW an attractive solution compared with some conventional configurations. Modeling and PWM control are addressed in this paper. Simulation and experimental results are presented.