Control Strategy to Mitigate Voltage Ripples in Droop-Controlled DC Microgrids

The commonly applied conventional voltage-mode droop control of a dc microgrid involves feeding the load current into the local voltage loop's reference via the virtual resistance loop. When the load currents include ripples, the power converters are forced to reproduce these ripples at their outputs. The state-of-the-art dc bus ripple mitigation techniques demonstrate a poor performance when added to a regular dc converter under the droop control since they essentially require the voltage ripples at the dc bus in order to generate the nonzero compensation currents. In this article, we propose the droop-based frequency-selective impedance control (dFSIC)—a primary level control strategy that is specifically designed for a regular droop-controlled dc converter to set the particular magnitudes of the total output impedance at the ripple frequencies. The values of the impedance magnitudes are specified in such a way that the distributed power converters can naturally share the ripple currents in proportion to their power ratings while the resulting impedance is kept low enough to constrain the dc bus voltage ripples. The effectiveness of the dFSIC and its capability to suppress voltage ripples at the multiple frequencies simultaneously, including the ones beyond the voltage loop's bandwidth, is demonstrated in a set of the experiments.