This paper proposes a novel integral sliding mode control (ISMC) scheme based on numerically solving a state-dependent Ricatti equation (SDRE), nonlinear feedback control for wind energy conversion systems (WECSs) with permanent magnet synchronous generators (PMSGs). Unlike the conventional ISMC, the proposed control system is designed with nonlinear near optimal feedback control part to take into account nonlinearities of the WECSs. The Taylor series are used to approximate the solutions of SDRE. More specifically, the nonlinear optimal feedback control has been obtained by solving continuous algebraic Ricatti and Lyapunov equations. Sliding variables are designed such that reaching phase is eliminated and stability is guaranteed. The proposed control method equipped with high-order observer can guarantee more superior results than linear techniques such as linear quadratic regulator (LQR), conventional ISMC, and first-order sliding-mode control (SMC) method. Increasing the number of terms of the Taylor's series of the proposed control law provides better approximation, therefore the performance is improved. However, this increases the computational burden. The effectiveness of the control method is validated via simulations in MATLAB/Simulink under nominal parameters and model uncertainties. INDEX TERMS Integral sliding mode control (ISMC), state-dependent Ricatti equation (SDRE), permanent magnet synchronous generator (PMSG), wind energy conversion system (WECS), variable-speed wind turbine, generalized high-order disturbance observer (GHODO), nonlinear output feedback, continuous approximation.

This paper focuses on designing a disturbance observer-based control (DOBC) system for PMSM drives. The cascade structure of the discrete-time PI-PI control system with tracking anti-windup scheme based on back-calculation algorithm has been designed for both loops. In this study, the high-order disturbance observer (HODO) based control has been proposed to improve the speed tracking performance of the control system for PMSMs. Note that the disturbance in the mechanical dynamics includes not only the load torque but also the torque losses resulting from the time-varying flux, hysteresis, and friction. The HODO observer is designed to estimate this total disturbance. Moreover, the proposed disturbance observer does not require the derivatives of the disturbance to be zero, like in the traditional ones. The proposed DOBC demonstrates its ability to improve speed tracking performance regardless of a load torque variation and the high-order disturbances caused by a cogging torque and a high-frequency electromagnetic noise in the PMSM system. The proposed disturbance observer-based control system has been implemented with a Lucas-Nuelle 300 W PMSM prototyping kit. In real-time experiments, the proposed algorithm with HODO achieves fewer speed errors and faster response comparing with the PI controller without HODO or PI controller with HODO of lower orders. The synthesis of the disturbance observer with discrete-time PI controller with the effective tractive anti-windup scheme has practical significance because it is widely used in the industry. The experimental results have proved the feasibility of the proposed control scheme.INDEX TERMS Anti-windup, disturbance observer based control, high-order disturbance observer, discrete-timePI controller, PMSM, back-calculation algorithm.

Recently, wind power production has been under the focus in generating power and became one of the main sources of alternative energy. Generating of maximum power from wind energy conversion system (WECS) requires accurate estimation of aerodynamic torque and uncertainties presented in the system. The current paper proposed the generalized high‐order disturbance observer (GHODO) with integral sliding mode control (ISMC) for extraction of maximum power via variable speed wind turbine by accurate estimation of wind speed. The assumption in previous works that considers the aerodynamic torque as slow‐varying is not applicable for the real system. Therefore, the high‐order disturbance observers were designed for precise estimation of uncertainties with fast‐changing behavior. A robust control system was designed to control the speed of the rotor at the optimal speed ratio. The obtained simulation results have shown the better performance characteristics than conventional linear quadratic regulator (LQR) approach. The stability of the proposed algorithm was proven by Lyapunov stability anaysis. Simulations results were obtained in Matlab/Simulink environment.

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In this article, integral servomechanism based state‐dependent Riccati equation (SDRE) nonlinear output feedback control for wind energy conversion systems (WECSs) has been proposed. The numerical off‐line solving the proposed SDRE control requires to formulate algebraic Riccati equation and algebraic Lyapunov equation. This method approximates the solution of SDRE with Taylor series expansions which are solved with MATLAB solvers. The maximum power point tracking is achieved by defining relation between the optimal angular shaft speed and maximum power providing optimal tip speed ratio in variable‐speed WECSs. The proposed control can significantly reduce the angular shaft speed errors of the permanent magnet synchronous generator (PMSG)‐based WECSs without use of disturbance observers to compensate model uncertainties, modeling errors and noise as in the conventional SDRE control method. The simulation results demonstrate the superior results under mean average errors and root mean square error evaluation methods of the angular shaft speed of PMSG. The performance of the proposed integral servomechanism based SDRE control system has been improved by 80.67% and 80.05% in both scenarios compared to the conventional SDRE control based on compensation technique with disturbance observers.

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