A New Voltage Balancing Technique for a Three-Stage Modular Smart Transformer Interfacing a DC Multibus

DC smart grids represent an alternative to traditional ac power distribution systems. The power conversion stage, interfacing the ac and the dc distribution systems, is the enabling technology to manage multiple dc loads and sources. The three-stage smart transformer (ST) is a promising solution, because it includes a dc-dc power conversion stage providing a dc multibus in output. The modular ST architecture supplies several dc outputs, which allow interfacing a dc smart grid with the ac power system. In this paper, the three-stage ST is based on a cascaded H-bridge (CHB) converter for the ac-dc power conversion, whereas dual active bridge (DAB) converters are adopted for the dc-dc power conversion stage. The peculiarity is that the dc voltage balancing is performed by the dc-dc power conversion stage instead of the CHB converter. It is advantageous for dc smart grids applications where the power sharing among the dc sources and loads is often not balanced. The design of the entire control system is based on the detailed small signal model of the ac-dc and dc-dc power conversion stages of the ST. High dynamic performance of the voltage balancing is fulfilled due to the commonly high switching frequency of the DAB converters.