Effects of yaw misalignment on the performance of a wind turbine

Causes of Yaw Errors

The measuring technology of wind turbines for wind direction determination is located on the roof of the nacelle downstream of the rotor. It is therefore not the direction of the undisturbed wind that is recorded, but the wind direction of the immediate caster. However, this has a twisted flow with strong turbulences imposed by the rotor, which is why manufacturers of wind turbines have to include corresponding correction functions in the turbine control. However, these correction functions would have to be adapted to the respective environmental conditions in order to take into account, for example, terrain influences and in particular the influence of neighboring systems. However, manufacturers do not make the necessary adjustments on site, which means that many systems are not correctly aligned. This problem is often exacerbated by incorrectly aligned wind vanes or incorrectly calibrated anemometers. The problem may already exist at the factory or be caused by service workers during operation, e.g. when working on the nacelle roof. However, the operator does not have the measuring technology to detect the corresponding errors, which results in significant performance losses and higher maintenance costs due to higher mechanical loads.

Determination of performance losses

If the rotor of the wind turbine is aligned perpendicular to the rotor plane, the flow area A corresponds to the area covered by the rotor. If, on the other hand, there is a nacelle malposition, the flow surface and thus the effective air mass flow is reduced by the factor cos(γ).

A model is often used which is based on the assumption that only the wind velocity component of the axial direction of flow contributes to the power conversion. Vector decomposition of the wind speed also results in a reduction of the speed component by the factor cos(γ). Due to the cubic dependence of the rotor power on the speed, the following functional relationship results from this consideration:

However, the cos³(γ) dependence derived from the simplified flow model does not reflect the aerodynamic effects of real flows, which influence the coefficient of power.

Measurements in the wind tunnel show a dependency of the power loss due to the fault position on an exponent of at least 1.80 and above[1][2][3].

Systematic investigation of wind farms shows that more than half of the wind turbines have a yaw misalignment of more than 4° [4].