*Article* **Analysis of Wind-Turbine Main Bearing Loads Due to Constant Yaw Misalignments over a 20 Years Timespan**

#### **Martin Cardaun \*, Björn Roscher, Ralf Schelenz and Georg Jacobs**

Center for Wind Power Drives, RWTH Aachen University, 52062 Aachen, Germany; bjoern.roscher@cwd.rwth-aachen.de (B.R.); Ralf.Schelenz@cwd.rwth-aachen.de (R.S.); georg.jacobs@imse.rwth-aachen.de (G.J.)

**\*** Correspondence: martin.cardaun@cwd.rwth-aachen.de

Received: 20 March 2019; Accepted: 8 May 2019; Published: 10 May 2019

**Abstract:** The compact design of modern wind farms means that turbines are located in the wake over a certain amount of time. This leads to reduced power and increased loads on the turbine in the wake. Currently, research has been dedicated to reduce or avoid these e ffects. One approach is wake-steering, where a yaw misalignment is introduced in the upstream wind turbine. Due to the intentional misalignment of upstream turbines, their wake flow can be forced around the downstream turbines, thus increasing park energy output. Such a control scheme reduces the turbulence seen by the downstream turbine but introduces additional load variation to the turbine that is misaligned. Within the scope of this investigation, a generic multi body simulation model is simulated for various yaw misalignments. The time series of the calculated loads are combined with the wind speed distribution of a reference site over 20 years to investigate the e ffects of yaw misalignments on the turbines main bearing loads. It is shown that damage equivalent loads increase with yaw misalignment within the range considered. Especially the vertical in-plane force, bending and tilt moment acting on the main bearing are sensitive to yaw misalignments. Furthermore, it is found that the change of load due to yaw misalignments is not symmetrical. The results of this investigation are a primary step and can be further combined with distributions of yaw misalignments for a study regarding specific load distributions and load cycles.

**Keywords:** wake steering; yaw misalignment; multi body simulation; main bearing loads; rain flow counts
