Optimal Output-Feedback Power Control for the Hybrid Excitation Synchronous Aero-Generator in High Wind Speed Operation

Abstract

This paper deals with the problem of controlling wind energy conversion systems operating at a high speed. We seek both the achievement of constant power operation and the minimization of Joule power losses in the aero-generator, which presently, is a hybrid excitation synchronous generator (HESG). Furthermore, the system safety over a wide range variation of the wind speed is taken into consideration in the control problem formulation. These energetic optimization and turbine safety requirements are achieved by using an output-feedback controller that is developed by nonlinear design techniques including a backstepping control, a high-gain observer and others. The controller consists of: (i) a sensorless online reference-speed optimizer designed to limit the extracted aerodynamic power in over speed mode; (ii) a power-limiter regulator based on the flux-weakening; (iii) an electromagnetic torque regulator ensuring power loss reduction in the HESG stator. The real-time implementation of the above objectives necessitates online measurements of the rotor flux, the rotor speed, the aerodynamic torque and the wind speed. The point is that the electromagnetic and the mechanical sensors are costly and entail reliability issues. Therefore, a state observer is designed to provide online estimates of all the required variables that are inaccessible to measurements. The output-feedback controller thus developed is formally analyzed using Lyapunov stability tools. The established theoretical performances are confirmed by a set of numerical simulations.