Resistive-Capacitive Output Impedance Shaping for Droop-Controlled Converters in DC Microgrids with Reduced Output Capacitance

In dc microgrids, droop control is widely employed in power converters interfacing distributed energy resources and common dc bus for automatic power sharing. In order to limit the dc-bus voltage variations even during load transient conditions, it is necessary to shape the output impedance of droop-controlled converters to be always lower than the dc resistive value. A commonly used way is to install bulky output capacitance, which not only increases the system cost, size, and weight, but also generates higher short-circuit fault currents. In order to avoid large output capacitance, this article proposes a design approach for droop-controlled converters, including the selection criterion of the output capacitance and the design of the droop coefficient. Herein, the required output capacitance is calculated based on the dc droop coefficient and the voltage control bandwidth. The droop coefficient is designed as a frequency-dependent term instead of a constant value. As a result, resistive-capacitive output impedance can be obtained with a much smaller output capacitance. The proposed design strategy is applied to buck and boost converters and validated by experimental results performed on a buck-based and a boost-based dc microgrid prototype, respectively.