Nonlinear PID control for pitch systems of large wind energy converters

The main control strategy of three-bladed, variable-speed, horizontal-axis large-sized wind turbines, in case of over rated wind speed is based on the equal actuation of the all three blades. In addition to this collective pitch control (CPC), the provided pitch angles can be slightly individually modified according to measurements of the root bending moments (IPC, individual pitch control) in order to mitigate loads. Finally, the pitch actuation produces additional tower oscillations that will require an active tower damping control (ATDC). All these control loops are usually PID-based. The present work describes the results obtained from a nonlinear PID control (NPID) approach for the pitch system of a large wind turbine. Nonlinear PID approaches based on hyperbolic secant functions are used for the CPC, ICP and ATDC, respectively. As simulation platform, the 5-MW reference turbine developed by NREL and the software FAST are used. Results show that the NPID approach can provide significant improvements in the control performance in comparison to the classic control approach, in particular when the wind speed goes far from its rated value.