A Frequency-Dependent Virtual Impedance for Voltage-Regulating Converters Feeding Constant Power Loads in a DC Microgrid

The power oscillations at the input of constant power loads (CPLs) fed by LC filters is a well-known phenomenon in dc microgrids. The phenomenon happens because of the negative resistance characteristic exhibited by CPLs. The oscillations can be damped by inserting virtual resistances at the output of voltage-regulating converters to offset the negative resistance of the CPLs. However, large virtual resistances lead to a poor voltage regulation of the dc bus. This paper proposes a frequency-dependent virtual impedance comprising of a parallel RL branch, which solves the issue of poor voltage regulation. Further, the large signal stability of the system in the presence of the virtual impedance is also investigated through the Brayton-Moser mixed potential analysis. Detailed simulations and analysis of the frequency-domain interactions of the system are carried out and the effectiveness of the proposed technique is showcased. Experimental results from a 250 W prototype CPL are included to validate the proposed technique.