Input Impedance Modeling and Reshaping Control Strategy for Hybrid Modular DC Transformers in DC Distribution Networks

The hybrid modular DC transformer (HMDCT), which combines the series resonant dual active bridge and phase shift dual active bridge sub-modules, has gained significant attention for its high efficiency and bidirectional power regulation capabilities. However, similar to conventional DC transformers, the HMDCT also faces the risk of instability due to the equivalent inductance of the input power line. To address this issue, this paper first proposes the HMDCT impedance model, which considers the individual input impedance of each sub-module and the effect of mutual coupling. Then, the impedance transfer function order reduction method and the corresponding hardware parameter requirements are given to simplify the stability analysis and guide the HMDCT design. Moreover, to improve the stability in the DC distribution networks, an impedance reshaping control strategy based on sub-module input voltage feedback is proposed. Experiments on a 500W HMDCT prototype are conducted to verify the theoretical claims.