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Robust Control of an Islanded Microgrid Under Unbalanced and Nonlinear Load Conditions

This paper presents a robust control strategy for the autonomous operation of a microgrid consisting of electronically coupled distributed generation (DG) units. The DG units are connected to a point of common coupling, and supply a load, which can be unbalanced and/or nonlinear. In practice, the load is usually unknown in terms of network topology and parameters. However, it is assumed that the load current is measurable and bounded. In this case, considering the load current as a measurable disturbance signal, the controller design is formulated to an H∞ optimization problem in order to minimize the adverse impact of harmonics and negative-sequence voltage due to nonlinear and unbalanced loads. The optimization problem is then converted into a convex linear matrix inequality (LMI) condition, which is simply solved using MATLAB LMI toolbox. The performance of the proposed controller is verified using hardware-in-the-loop (HIL) real-time simulations carried out in OPAL-RT technologies. The HIL results show that the proposed controller provides the load with a set of sinusoidal, three-phase balanced voltages despite several unbalanced and nonlinear load conditions.