Implementation of Bidirectional Resonant DC Transformer in Hybrid AC/DC Micro-Grid

Abstract

As a reliable device for voltage regulation and isolation, a line frequency transformer is commonly connected in series with bidirectional converter (BIC) to interlink the ac and dc networks of hybrid ac/dc microgrid. However, it may not be suitable for the applications where weight and space occupation are of important concerns such as hybrid ac/dc microgrid. In this paper, bidirectional resonant dc transformer (BRDT) is proposed to replace the conventional bulky transformer for bus voltage matching and galvanic isolation. In order to simplify systematic coordination with BIC and central controller, BRDT is designed as an ideal transformer with simple open-loop scheme. 50% duty ratio scheme is implemented as it has been proven to be better than the phase-shift scheme in order to ensure BRDT conversion efficiency. A generalized model for various BRDT topologies, including CLLC, CLL, and LLC high frequency transformer (HFT), is established for analysis. The impacts induced by the transformer leakage, magnetizing inductances, and extra resonant capacitors are considered when designing the HFT. The conversion gain of the designed BRDT has also been analyzed in the full power range. Lab-scale prototypes for BRDT and hybrid ac/dc microgrid have been developed for experimental verifications. The performances of BRDT and system in both steady and transient states have been confirmed.