Sliding mode control-based active power control for wind farm with variable speed wind generation system

Active power control in wind farm, implemented with hierarchical structure (including the central wind farm active power control system and the control system of variable speed wind generation system), has significantly improved the wind-farm-generated power. However, the interaction between the control system of variable speed wind generation system and the central wind farm active power control system has not been considered in the control system design of both sides. The wind generation system with active power control in wind farm, as a high-order nonlinear large-scale system, will not only autonomous operating with maximum power capture, but also being demanded to regulate power output according to the power set points ordered by the central wind farm active power control system. Therefore, robust active power control of wind farm is clearly necessary. In this article, sliding mode is used to design the active power control system for wind farm, which makes the output power of wind farm track the active power order from the power grid operator asymptotically. Particularly, the control system of the variable speed wind generation system is designed with cascaded structure: the input of the internal rotational speed control loop of the wind generation system is controlled by the external power loop which tracks the specified power set point. Then, based on the dynamic model of wind farm, the central wind farm active power control system is designed to control the total output power of wind farm by using the sliding mode control method. Furthermore, in order to ensure the given power set point is within the regulation capability of the wind generation system, the proportional distribution strategy is utilized to specify the power set point according to the available output power of each wind generation system, which is quantified by ultra-short term power prediction model. The proposed approach is illustrated by implementing it to the active power control of a wind-farm model in simulation. The simulation results demonstrate that the proposed method outperforms the method based on proportional-integral control.