A Robust Two-Degree-of-Freedom Control Strategy for an Islanded Microgrid

This paper presents a new robust control strategy for an islanded microgrid in the presence of load unmodeled dynamics. The microgrid consists of parallel connection of several electronically interfaced distributed generation units and a local load. The load is parametrically uncertain and topologically unknown and, thus, is the source of unmodeled dynamics. The objective is to design a robust controller to regulate the load voltage in the presence of unmodeled dynamics. To achieve the objective, the problem is first characterized by a two-degree-of-freedom (2DOF) feedback-feedforward controller. The 2DOF control design problem is then transformed to a nonconvex optimization problem. Furthermore, the nonconvex optimization problem is reduced to a convex linear matrix inequality-based optimization problem which can be easily solved. To achieve optimal performance for the system, unlike the most conventional 2DOF design approaches, the feedback and feedforward controllers are jointly designed. Finally, simulation case studies performed in the MATLAB/SimPowerSystems Toolbox show that the proposed control scheme is strongly robust against uncertainties in the load parameters and against the unknown dynamics.