Reduction of Output Capacitance for DAB DC-DC Converters in DC Microgrids by Shaping Resistive Output Impedance

The reduction of output capacitance for Distributed Energy Resource (DER) converters in DC microgrids is expected, for the purpose of saving cost, decreasing system weight and size, and facilitating the short-circuit fault isolation. However, the decrease of output capacitance leads to the increase of output voltage spike/dip during load changes, and the output voltage may go out of the acceptable range during transient. From this point of view, the resistive output impedance is a better design solution for DER converters than non-resistive solutions, because it allows a larger voltage spike/sag and thus enables the use of smaller output capacitance. This paper proposed a design methodology for droop-controlled Dual Active Bridge (DAB) dc-dc converters, so that the resistive output impedance can be achieved. This design approach consists of the selection of output capacitance and the design of droop controller. Besides, for converters with small output capacitance, the output voltage presents high switching ripple. In such a case, the traditional single sampling technique causes a steady-state error between the averaged output voltage and the sampled one, impacting on the output impedance. This issue is addressed in this paper by a hybrid sampling method which combines the instantaneous value and the well-filtered value of the output voltage. Finally, the proposed design method with the hybrid sampling method is verified by experimental results.