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Apparent Impedance-Based Adaptive Controller for Improved Stability of a Droop-Controlled Microgrid

Droop control is the most common approach for controlling microgrids (MGs) and interfacing distributed generators (DGs) due to its inherent power-sharing characteristics and low dependence on communication systems. However, the stability of MGs operated with this control paradigm is sensitive to the droop parameter values and system topology. This situation will be even more challenging in future grid scenarios, where system reconfiguration and parameter uncertainty will become more relevant. In this article, a centralized controller for improving the stability margins of an MG is proposed. First, the apparent impedance of the MG is periodically identified by injecting a multitone current perturbation. Then, the matrix fitting technique is applied to calculate the system eigenvalues based on the estimated apparent impedance. Depending on the location of the dominant eigenvalues, the droop gains are modified by using a control law to ensure sufficient stability margins. No previous information of the system topology or parameter values is required. It is shown that this method can greatly improve stability margins without compromising the inherent power sharing, transient performance, and low dependence on communication systems of the droop control. Simulation and experimental results for an MG based on DGs rated at 60 kVA are included to validate the proposed control scheme.