Simple model for describing and estimating wind turbine dynamic inflow

Wind turbines operate with sudden change in pitch angle, rotor or wind speed. In such cases the wake behind the turbine, achieve steady state conditions only after a certain delay. This phenomenon is commonly called dynamic inflow. There are many models for dynamic inflow. The most accurate use a method that can be characterised as the blade element momentum method plus a dynamic equation for the induction factor. This method then needs calculations along the blade for a number of sections including numerical solution of equations. This is numerical demanding. The simplest models amounts to placing a lead-lag filter after rotor torque and thrust calculated from static tables of the power and thrust coefficients. The filter constants will then vary with average wind speed. The filtered versions of torque and thrust are then an approximate modelling of the dynamic inflow. The dynamic inflow model suggested here places itself in between the most complex and the most simple both in accuracy, numerical demands and physical appeal. The suggested models behavior is demonstrated by simulation and the usefulness for extended Kalman filtering is assessed both via simulated data and real full scale turbine data.