A self-switched virtual impedance based stabilization method for a droop controlled DC microgrid with Constant Power Loads and input load filters

Abstract

Tightly controlled power electronic converters connected to a DC microgrid behave as Constant Power Loads (CPLs) and they have the potential to induce voltage oscillations at the DC bus. These oscillations become more probable in the presence of lightly damped L-C filters at the input of CPLs. The oscillations can be damped by inserting appropriate virtual impedances in the DC microgrid. It has been observed that virtual impedances inserted at the inputs of CPLs tend to affect their performance. Therefore an active damping technique is proposed in this paper which inserts virtual impedances at the outputs of the energy storage systems (ESSs) of the DC microgrid. The virtual impedance is effective at the resonant frequency of the input filter where damping is particularly required. This gives room to accommodate droop control at low frequencies in the ESSs. Stability analysis using Middlebrook impedance criteria and Eigen value migration plots are presented. The limits of the magnitude of the proposed virtual impedance are calculated using root loci and Routh Hurwitz criteria. Simulation results are presented to verify the proposed strategy.