Nonlinear Optimal Control for PMSG-Based Wind Energy Conversion Systems

This paper presents the design of a nonlinear optimal control strategy for a wind energy generation system based on a small scale wind turbine, a permanent magnet synchronous generator, a back-to-back power converter, and an LCL filter interconnected with the grid. The main control objectives in this energy system involve the trajectory tracking and regulation of different system variables as generator angular speed, DC-link voltage and reactive power. In order to fulfill these objectives, the application of optimal controllers based on the state-dependent coefficient factorization technique is proposed for both the generator-side converter and the grid-side converter. The advantages of the proposed control scheme are to achieve a wide system operating range, due to the controller design takes into account the system nonlinearities, and in addition, an efficient trajectory tracking of time-varying references generated by a maximum power point tracking algorithm, which uses a wind speed estimator. Also, an LCL filter is used to attenuate the harmonic content of the injected current to the utility grid to satisfy interconnection standards, which requires a THD below 5%. The effectiveness of the proposed controllers is verified via simulations developed in Matlab/Simulink, where the variables to be controlled reach the desired references that ensure proper operation, maximum power extraction, and fulfillment of interconnection standards.