Robust model reference control for coordinated voltage and frequency regulation in islanded microgrids

Primary control of distributed generators in an islanded microgrid alone is insufficient to stabilize system voltage and frequency to their nominal values; therefore, an additional secondary control strategy is essential. This paper presents a distributed Model Reference Control strategy as secondary control for stabilizing the voltage and frequency of an islanded microgrid. The proposed approach of controller design of a distributed generator relies solely on information from neighboring distributed generators, which communicate with their immediate neighbors via a communication network. The distributed control techniques for microgrids often require detailed system modeling, including line characteristics, load profiles, and parameters of other distributed generators, many of which may be unknown or difficult to determine. In contrast, the proposed control scheme considered the unmodeled dynamics, external disturbances, and uncertainties of the system while designing the controller. The proposed technique employs a reference model, using cooperative tracking errors to generate a virtual reference for each follower, which forms the basis of the main control strategy. Employing cooperative tracking and disagreement errors improves robustness against external disturbances and system uncertainties. Consequently, the terminal voltages of the distributed generators are restored to their prescribed reference values, the operating frequency is regulated to 50 Hz, and proportional active-power sharing is achieved. A detailed stability analysis demonstrates that the disagreement error is uniformly ultimately bounded, ensuring that each follower synchronizes to the leader’s state within bounds. Simulation studies on an islanded microgrid test system verify the robustness and effectiveness of the proposed control scheme under load variations, matched uncertainties, and parameter perturbations.