A Hybrid Modular DC Solid-State Transformer Combining High Efficiency and Control Flexibility

This article presents a hybrid modular dc solid-state transformer (HMDCSST) composed of a series resonant dual active bridge (SR-DAB) and a phase-shift dual active bridge (PS-DAB), aiming at improving the transfer efficiency as well as maintaining the control flexibility key problems in terms of modeling and control strategy are discussed in this article. The generalized average and small-signal models of the HMDCSST are derived and confirmed by simulation, in which two LC filters are also considered to reduce the ripple components of the input and output currents. Based on the models, the control strategies to achieve the flexible control of the output voltage and power are designed. Moreover, as a key problem for HMDCSST, the design principle of the number for each type of DAB is discussed to meet the demands of the grid. Finally, a three-module prototype of HMDCSST consisting of one PS-DAB and two SR-DAB modules was built-up and tested, and the results proved that HMDCSST has higher efficiency than the traditional DCSST based on PS-DABs purely, without sacrificing the output voltage and power regulating capability in the meanwhile.