Robust Sliding Mode and Mixed $H_2$/$H_\infty$ Output Feedback Primary Control of AC Microgrids

In this article, a unified, robust hierarchical control structure is proposed for operating networked multi distributed generation grid-forming power converters in islanded alternating current (ac) microgrids (MGs). The primary control level stabilizes the frequency and the voltage of an ac MG through a cascaded structure, consisting of a droop controller, a mixed $H_2$ / $H_\infty$ -based voltage controller, and an adaptive backstepping integral nonsingular fast terminal sliding mode control (ABINFTSMC) for current loop. In the inner control loop, an adaptive robust nonlinear controller is designed for regulating and tracking of the current reference in the presence of unknown bounded uncertainties and external disturbances. The outer loop controller design problem is formulated by a set of linear matrix inequalities and then solved as a multiobjective optimization problem, by using a fuzzy decision-making tool (FDMT) to provide the best tradeoff solution. Besides, the conventional distributed protocol has been used in the secondary layer to compensate for the deviation caused by the primary layer. Finally, the effectiveness of the proposed control scheme is evaluated through offline time-domain simulation studies based on different scenarios in the MATLAB/Simulink environment for a test MG system.