Field of the Invention

This invention relates to an improved method of operating a wind turbine to reduce fatigue of parts therein.

Background to the Invention

A problem with wind turbines is that the rotor blades are subjected to cyclical stress and strain as the rotor turns, which in turn causes wear and fatigue on its parts.

Gravitational forces due to the blade's own mass mean that when the blade is at the 3 o'clock, or 90 degree azimuth, position, the components on one side of the blade, normally the trailing edge will be in tension and the components on the opposite side, normally the leading edge, will be in compression. By contrast, when the blade is at the 9 o'clock, or 270 degree azimuth, position, the regions tends to be reversed. In this manner it can be seen that during a complete rotor cycle the contributory levels of stress and strain on the blade components as a result of gravity will see a nominal sinusoidal variation in magnitude.

Vertical wind shear, which is the condition where wind speeds (generally) increase according to the height above ground level, means that when a blade is at its highest position (12 o'clock) it will experience forces due to wind and aerodynamic lift that are greater than at its lowest position (6 o'clock). Therefore, during a complete rotor cycle the contributory levels of stress and stain on the blade components as a result of wind shear will see a nominal sinusoidal variation in magnitude.

Horizontal wind shear, which may be caused by local terrain, causes wind speed variations substantially horizontally across the rotor and results in the wind turbine blades experiencing peak stress and strain at the 3 o'clock and 9 o'clock positions, caused by both horizontal wind shear and aerodynamic lift. Again, this results in a nominal sinusoidal variation in the magnitude of the stress and stain on the blade components.

Yaw misalignment, which is the condition wherein the rotor is not orientated

perpendicularly to the nominal wind direction, results in the angle of attack - the angle by which the wind is perceived by the blade to arrive at the blade surface and so cause aerodynamic lift - and apparent wind magnitude - the magnitude of the wind speed as perceived by the blade, which is a combination of true wind speed, wind angle and blade speed - varying when the blade is close to its highest position (12 o'clock) compared to when its close to its lowest position (6 o'clock). It can be seen that during a complete rotor cycle, the operation of the blade will vary such that the contributory levels of stress and strain on the blade components as a result of yaw misalignment, that is the varying forces that are both a direct result of the incoming wind and the aerodynamic lift, will see a nominal sinusoidal variation in magnitude.

The above different forces acting on the rotor blades of a wind turbine will typically be out of phase. For example, the gravity induced forces typically show a minimum at the 3 o'clock position, or 90 degrees azimuth, and a maximum at the 9 o'clock position, 270 degrees azimuth, whereas the vertical wind shear typically shows a minimum at the 12 o'clock position, or 0 degrees azimuth, and a maximum at the 6 o'clock position, or 180 degrees azimuth. Similarly, the phase of yaw misalignment can also induced forces typically show a minima when the rotor is close to the 12 o'clock position, or 0 degrees azimuth, and a maxima at the 6 o'clock position, or 180 degrees azimuth. These cyclical components can add constructively or destructively and result in an overall sinusoidal-like cyclic stress and strain component on each of the blades, the pitch control system, pitch and rotor bearings, the rotor, the tower, the drive train and various other components of the wind turbine, including the generator. This increases the fatigue on the parts and thus increases the chances of premature failure of the turbine.

Where wind turbine generators employ individual pitch control, independent pitch control or other dynamic pitch mechanisms, the cyclic forces can cause the pitch control system to attempt to react to the forces in an equally cyclic manner. When operating in this manner, the pitch system components and bearings undergo cyclic fatigue inducing forces, as the pitch system tries to counter-act the impact of the Wind, Gravity and Operating regime induced forces.

One method for reducing the stress and strain components at above rated power is to adjust the pitch angle of the individual rotor blades. Above rated power is the time when the forces become particularly significant to the wind turbine.

Summary of the Invention Accordingly, the present invention is directed to a method of reducing the stress load in a wind turbine rotor comprising the steps of:

determining the wind direction; and

yawing the wind turbine rotor such that the plane of the rotor blade is substantially non- perpendicular to the wind direction so that cyclic stress components are induced in the wind turbine rotor that are out of phase with the inherent natural stress components, thereby reducing the amplitude of the overall stress load in the wind turbine.

The wind turbine rotor is intentionally misaligned, or moved from perpendicular to the nominal wind direction, in order to introduce a cyclical yaw misalignment stress and strain that is out of phase from the other forces acting on the wind turbine. The magnitude of the yaw misalignment forces can be adjusted according to the level of yawing applied. By inducing out of phase cyclic stresses that combine destructively with the natural stress components, the overall stress in the components can be reduced. The amount of yawing can be determined according to the prevailing conditions by calculating the phase and level of the stresses induced according to each of the causing factors, or by looking up the yaw levels from a look-up table. The look-up table may be calculated from analysing data obtained from previous operation of the wind turbine. The determination of the stress in the rotor blade may be undertaken using optical fibre strain sensors.

The result of intentional operation with a yaw misalignment is to reduce the magnitude of the cyclical stress and strain forces on the components, which will consequently reduce the fatigue and loading on the turbine components. This in turn helps to increase the lifetime of components, reduce the frequency of components failing and maximise the availability of the turbine. Additionally, by reducing the magnitude of the cyclical stress components, the extent of travel and loading of the pitch system components can also be reduced as a result of the yaw misalignment. This, in turn, assists with extending the operational life and also improves the performance of any independent pitch control and individual pitch control that might be present, particularly if the pitch control system would otherwise be at the extent of its operating speed, power or torque characteristics.

It is advantageous that the wind turbine is operating at above rated power. The stress and strain components will be their greatest at above-rated power; therefore, the present invention is particularly useful in above-rated power applications. As the production capacity is at a maximum in such conditions, it is particularly advantageous to apply the present invention at above rate-power situations. This reduces the risk of fatigue and wearing of parts in the wind turbine.

It may be desirable to have a mechanism to turn off the method of controlling the wind turbine according to the present invention in situations where the wind turbine is operating at below rated power. This is because yaw misalignment in below rated power conditions may reduce the level of power production, and thus revenue. However, at below rated power, the forces impacting on the turbine will be low, thereby reducing the need to further reduce the stress and strain.

Preferably, the method further comprises the steps of:

Determining the wind shear across at least one wind turbine blade;

Determining the degree of yawing required to induce an out of phase stress component sufficient to reduce the overall stress in the rotor blade; and

Yawing the wind turbine rotor by the determined amount. The destructive addition of cyclical stress forces of sufficient magnitude to 'cancel' at least part of the natural forces reduces the fatigue and wear on the parts of the wind turbine. Taking into account the wind shear enables one to determine a more accurate level of the yawing that is required to reduce the stress in the turbine. It may be desirable to adjust a dynamic pitch control system, where one exists, to increase or decrease the lift over the rotation cycle of the rotor, in order to compensate for any lost power due to the yawing effect. Additionally, or alternatively, the pitch of the blades may be dynamically adjusted to assist in introducing the out of phase forces. In a preferred construction, once the wind turbine rotor has been yawed to a non- perpendicular position, the pitch of the rotor blades is adjusted to increase the power input into the wind turbine rotor.

When the rotor has been yawed, the otherwise detrimental impacts on power production capacity as a consequence of yaw misaligned operation should not manifest. Additionally, any possible reduction in power from yaw misalignment may be positively recovered by adjustment of the blade pitch angle or other turbine regulatory system. This is particularly advantageous in when operating above rated power, wherein the wind generating capacity is already beyond the requirements of the generator. Triggering operation of the system in above and below rated power may be automated and so turned on in suitable conditions to reduce fatigue on parts when required.