Analysis of the effect of curtailment on power and fatigue loads of two aligned wind turbines using an actuator disc approach

To study the effects of curtailment on both power production and fatigue loading, actuator disc (ACD) simulations of two turbines aligned in the wind direction are performed with the EllipSys3D code developed at DTU/Risø. Blade forces, extracted along a line that rotates in the rotor plane with the rotational velocity of the respective turbine, are used to calculate flapwise bending moments. After applying a rainflow counting algorithm an equivalent moment is calculated. Power curtailment is introduced by changing the pitch angle of the first turbine. Evaluation is made with regards to load reduction at the second turbine and to total production loss compared to the situation without any curtailment. The aeroelastic code Vidyn, developed by Teknikgruppen (Ganander [20]), is used to validate the above-mentioned ACD-based load calculation method. For this purpose, the EllipSys3D simulations are also performed without the second turbine and time series of cross sectional velocity fields are extracted at positions corresponding to the placement of the downstream turbine tested above. These fields are later used as an input for the simulations performed with Vidyn. The results from Vidyn and the results based on the ACD blade forces show similar trends. Fatigue loads at the downwind turbine are clearly decreasing as the blade pitch angle of the upstream turbine is increasing. The achievable amount of fatigue load reduction depends on the level of the imposed pre- generated turbulence as well as the spacing between the turbines. The presented method is intended for further development of wind park optimization strategies