Investigation of Load Reduction Possibilities in Wind Turbines Using a Fluid Power Pitch System

The pitch system is a central part of modern wind turbines and good pitch control is essential for proper operation of the wind turbine. However, often when considering the pitch control, the pitch system dynamics is approximated by a simplified low order model, which may be acceptable for electrical pitch systems and turbine control purposes, but which does not capture the potential damping possibilities that may arise by actively using the fluid power systems to reduce the loads on the wind turbine structure. The focus of the current paper is therefore on the load reduction possibilities arising from applying an active damping filter in the fluid power systems to damp structural loads on the wind turbine, while maintaining the power production. Utilising the 5MW NREL reference wind turbine the paper presents a model of the fluid power pitch system, which is incorporated into the aeroelastic code FAST. Based on the model, an active damping approach is applied in combination with the standard pitch control to reduce oscillations in the pitch actuator force and hence the fatigue loadings on the mechanical structure. With basis in the implemented algorithm, the wind turbine is simulated under standard (IEC) load conditions and the load reduction possibilities analysed for the critical areas of the wind turbine along with performance of the wind turbine (pitch angle and power output).