Power-based droop control suppressing the effect of bus voltage harmonics for DC microgrids

This paper presents a power-based droop controller for distributed energy resources converters in dc microgrids that are connected to the utility ac grid by a grid-interface converter. Typically, when dc microgrids are interfaced to ac power systems, the microgrid dc bus voltage can be affected by fluctuations caused, for example, by background harmonics present in the ac systems or by fluctuating power absorptions, as is the case, for example, with single-phase rectifiers. By means of notch filters included in the converter controller, the solution proposed herein allows DERs converters to track given power references without being affected by dc bus voltage fluctuations. However, due to the high attenuation and the phase lag induced by notch filters, instability issues may arise. For this reason, a modified notch filter with a smaller phase lag is proposed. Moreover, an output current feedforward method which also enhances the system stability is presented. Experimental results from a dc microgrid prototype composed of four DERs converters and a grid-interface converter, all with rated power of 5kW, are reported.