Three-Step Switching Frequency Selection Criteria for Symmetrical CLLC-Type DC Transformer in Hybrid AC/DC Microgrid

In a hybrid ac/dc microgrid, the symmetrical CLLC-type dc transformer (CLLC-DCT) generally operates at the resonant frequency with the simple and low-cost semi-regulated open-loop scheme to ensure the power transmission (PT). However, more than one resonant frequency may exist, and they feature different characteristics for DCT. Meanwhile, the practical resonant frequencies are varying with temperature and power, which will degrade the PT ability if the switching frequency selection is not considered. Therefore, a switching frequency selection criteria (SFSC) approach is put forward for the symmetrical CLLC-DCT. For convenience, the active PT ratio (APTR) is employed as evaluation index of SFSC. First of all, the number and accurate values of resonant frequency are achieved in Step I. Then, in Step II, the optimal resonant frequency is determined as APTR-based criterion. In Step III, the parameter-variation-based criterion is presented to determine switching frequency. Finally, the experimental results verify that the semiregulated DCT based on the proposed approach not only improves efficiency, but also achieves the approximate PT ability with the relatively high-cost closed-loop controlled DCT.