Switching control of wind turbine
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Modern variable speed wind turbines operate within large range of wind speeds. Control of such a turbine is partitioned in two modes: a low wind speed mode and a high wind speed mode. Depending on the prevailing wind speed and the power demand from grid, a turbine is controlled to switch between these two modes. This work studies the stability of this switched system with two different methods. For the first method, linearized approximations of the closed loop wind turbine system are derived, which are later used to show that the system has a Common Quadratic Lyapunov Function (CQLF) in both control modes under certain switching conditions. The existence of CQLF establishes stability criteria for the system. Second stability method investigates input-output stability of the nonlinear switched system and shows the system to be L2 stable with strong gain. During mode switching, critical dynamics of the turbine such as the output power and mechanical loads can experience steep, large changes in some situations even though the switching action is stable. A new control technique is presented in this work to address this issue of volatile dynamics during turbine switching while preserving the original system structure and stability results. Then, performance of the wind turbine system is analyzed under various wind conditions and a method to select appropriate control gain values is discussed.