Impedance modeling and verification of a dual active bridge (DAB) DC/DC converter enabled DC microgrid in FREEDM system

This paper investigates the terminal impedance characteristics of the dual active bridge (DAB) DC/DC converter enabled DC microgrid. In this system, the DAB converters are employed not only in the solid state transformer (SST) to enable the DC microgrid, but also to integrate the distributed energy storage device (DESD). Although the same topology is used, the output impedance of the DAB converters are different from each other due to various control schemes utilized, thus different effect on system stability. In addition, the optimized system power management strategy requires mode transition of the DESD, which also results in different output impedance. Therefore, the output impedance of the DAB converters together with the rear-end inverter are derived considering the aforementioned control flexibility. The obtained analytical impedance information is verified by comparing with the measured results from circuit perturbation. In addition, the impedance characteristics of each DAB converter is analyzed, which can be further used to evaluate system stability, and formulate a system-level stability-oriented design methodology.