DK201670818A1 - A method for controlling noise generated by a wind turbine - Google Patents

Abstract

A method for controlling a wind turbine is disclosed, the wind turbine comprising a set of wind turbine blades (1), each wind turbine blade (1) being provided with at least one air deflector (2) being movable between an activated position in which it protrudes from a surface of the wind turbine blade (1) and a de-activated position. Noise constraints for the wind turbine under the prevailing operating conditions are obtained, and an air deflector setting enabling the wind turbine to be operated within the obtained noise constraints is determined. The wind turbine is operated while applying the determined air deflector setting.

Description

A METHOD FOR CONTROLLING NOISE GENERATED BY A WIND TURBINE

FIELD OF THE INVENTION

The present invention relates to a method for controlling noise generated by a wind turbine, in particular controlling air deflectors arranged on the wind turbine blades of the wind turbine.

BACKGROUND OF THE INVENTION

Modern wind turbines are controlled and regulated continuously with the purpose of ensuring optimal power extraction from the wind turbine under the current wind, and weather, while at the same time ensuring that the loads on the different components of the wind turbine are at any time kept within acceptable limits, and while respecting any externally set operational constraints. Such external constraints may include noise constraints, i.e. constraints regarding a noise level which the wind turbine is allowed to generate under a given set of operating conditions. The noise constraints may vary depending on time of day, wind direction, wind speed, etc.

Modern wind turbines are often controlled based on pitch actuation which regulates the aerodynamic properties of the wind turbine blades. By pitching the blades, it is possible to control the lift and drag experienced by the wind turbine blade. It is normal practise to pitch the blades according to some pitching strategy, in order to obtain a desired power production and load level for the wind turbine. In order to provide further possibilities for controlling the aerodynamic properties of a wind turbine blade, a number of concepts have been proposed, including so-called air deflectors. Air deflectors are elements configurable to be in an activated state where they are pushed out of the blade to alter the aerodynamic properties of the blade, and in a de-activated state where a top portion of the air deflector forms a portion of the surface of the rotor.

An example of an air deflector system is provided in US 8,192,161.

Wind turbines can emit noise, with aerodynamic noise sources including separated/stall flow noise, trailing edge noise, laminar boundary layer vortex shedding noise, tip noise, noise from surface imperfections, blade rotation noise, lifting/control surface loading noise, noise from the interaction between the blades and the wake vortex, noise from the interaction of the blades with atmospheric turbulence, and noise from the blade passing the wind turbine tower. Wind turbines also produce mechanical noise, for example from the gearbox.

Aerodynamic noise production is highly dependent on the relative velocity of the wind and the wind turbine blades. A faster relative velocity results in a greater production of noise. For this reason, simply reducing the blade velocity by restriction or reduction of the RPM of the wind turbine has previously been used for controlling the noise of wind turbines. This is generally an effective method, but it results in a reduced power output. Similarly, the blade pitch can be increased in order to reduce the blade load, again reducing noise production but also reducing power output.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a method for controlling a wind turbine comprising blades equipped with air deflectors to be operated within given noise constraints.

The invention provides a method for controlling a wind turbine, the wind turbine comprising a set of wind turbine blades, each wind turbine blade being provided with at least one air deflector being movable between an activated position in which it protrudes from a surface of the wind turbine blade and a de-activated position, the method comprising the steps of: - obtaining noise constraints for the wind turbine under the prevailing operating conditions, - determining an air deflector setting enabling the wind turbine to be operated within the obtained noise constraints, and - operating the wind turbine while applying the determined air deflector setting.

The invention provides a method for controlling a wind turbine. The wind turbine comprises a set of wind turbine blades. During operation of the wind turbine, the incoming wind acts on the wind turbine blades, thereby causing a rotor to rotate. The rotating movements of the rotor are transformed into electrical energy by means of a generator. The rotor may be connected to the generator via a gear arrangement, or it may be connected directly to the generator. The latter case is sometimes referred to as a direct drive wind turbine.

Each wind turbine blade is provided with at least one air deflector being movable between an activated position in which it protrudes from a surface of the wind turbine blade and a deactivated position. In the de-activated position the air deflector is not protruding from the surface of the wind turbine blade, and it may advantageously form a portion of the surface.

In the present context the term 'air deflector' should be interpreted to mean a device being configured to extend from a surface, in particular a surface of a wind turbine blade, in order to modify the air flow across the surface, when being in an activated state. Thereby, activating an air deflector causes a change in the aerodynamic properties of the surface, for instance affecting the lift and/or the drag of a wind turbine blade, and/or affecting a boundary layer along the surface. This may, e.g., result in an increase or a decrease of power production of the wind turbine, and/or in a decrease in loads on the wind turbine, thereby minimising the risk of potential damage to components of the wind turbine.

The air deflectors may be arranged on the wind turbine blades at various positions along a chordwise direction, such as at or near a leading edge, at or near a trailing edge and/or at a mid-chord position. Furthermore, air deflectors may be arranged on a suction side of the wind turbine blade and/or on a pressure side of the wind turbine blades. In an embodiment, the air deflector is placed on the suction side. Such air deflectors may upon activation diminish the lift of the blade.

Each wind turbine blade may comprise a plurality of air deflectors, e.g. distributed along a lengthwise direction of the wind turbine blade. In this case the air deflectors may be adapted to be activated and de-activated individually, i.e. at a given point in time some of the air deflectors may be in the activated position while other air deflectors are in the de-activated position.

According to the method of the invention, noise constraints for the wind turbine under the prevailing operating conditions are initially obtained. The noise constraints define limits for the noise which the wind turbine is allowed to generate. The obtained noise constraints depend on the prevailing operating conditions. For instance, the noise constraints may depend on wind and weather conditions, such as wind speed, wind direction or precipitation. A higher noise level may be acceptable when the wind speed is high and/or when precipitation, such as rain, snow or sleet is present, since in this case noise generated by other noise sources is expected to be at a high level. Furthermore, the noise constraints may be stricter when the wind direction is towards noise sensitive neighbours than when the wind direction is in an opposite direction. Alternatively or additionally, the noise constraints may depend on the time of day, for instance imposing stricter noise constraints during the night than during the day. Alternatively or additionally, the noise constraints may depend on the control of one or more neighbouring wind turbines, e.g. including a noise level or an expected or estimated noise level of such neighbouring wind turbines. This is in particular relevant if the wind turbine is arranged in a wind farm.

The noise constraints may be provided to a wind turbine controller from an external source, such as a power plant controller, also called a wind farm controller, or a surveillance station. As an alternative, the noise constraints may be derived from information regarding the prevailing operating conditions, possibly in combination with a required output power level to be delivered by the wind turbine. As a further alternative, the noise constraints may be derived by using a noise model which models the noise level at a given emission point. Such a noise model may be calculated online by the wind turbine controller, by the power plant controller or by another external computing unit.

Next, an air deflector setting is determined, which enables the wind turbine to be operated with in the obtained noise constraints. When a wind turbine is operated with one or more air deflectors in the activated position, the noise generated by the wind turbine may be higher than the noise generated when the wind turbine is operated with all air deflectors in the deactivated position. Furthermore, the noise level may increase as the number of activated air deflectors increases, and it may further depend on which air deflectors are in the activated position and which are in the de-activated position. The noise level may also depend on the wind speed, the rotor speed and/or other ambient conditions. On the other hand, activating the air deflectors may be desirable in order to fulfil other objectives during operation of the wind turbine, e.g. in the form of fulfilling load requirements, minimising wear on various parts of the wind turbine, optimising power production, etc. It is therefore desirable to select an air deflector setting which at least partly fulfils such objectives, but which still prevents that the noise generated by the wind turbine exceeds the noise limits defined by the noise constraints.

Finally, the wind turbine is operated while applying the determined air deflector setting. Since the determined air deflector setting is selected in such a manner that it enables the wind turbine to be operated within the obtained noise constraints, it is thereby ensured that the noise limits defined by the obtained noise constraints are not exceeded. On the other hand, activation of at least some of the air deflectors is possible, thereby allowing other control objectives to be fulfilled.

The method may further comprise the step of estimating and/or measuring the noise which is actually generated by the wind turbine, in order to ensure that the wind turbine is actually operated within the noise constraints.

The step of determining an air deflector setting may comprise determining an air deflector setting which results in a specific control objective to be reached, unless the air deflector setting causes the wind turbine to operate outside the obtained noise constraints. As described above, such control objectives could, e.g., include fulfilling load requirements, minimising wear on various parts of the wind turbine, such as blade pitch bearings or wind turbine blades, optimising power production and/or any other suitable control objective. According to this embodiment, an air deflector setting may initially be determined, which ensures that a desired control objective is fulfilled, without taking noise constraints into consideration. It may then be investigated whether or not this air deflector setting will also result in the wind turbine being operated within the obtained noise constraints. If this is the case, the initial air deflector setting is simply selected. However, if this is not the case, an air deflector setting where the air deflectors are activated to a less extent is selected instead, in order to ensure that the wind turbine will be operated within the obtained noise constraints. Thus, according to this embodiment, the air deflectors may be activated to the greatest possible extent, as long as it is ensured that the wind turbine will not be caused to operate outside the obtained noise constraints.

The method may further comprise the steps of: - determining a pitch angle setting for the wind turbine blades, the pitch angle setting enabling the wind turbine to be operated within the obtained noise constraints, and - operating the wind turbine while applying the determined pitch angle setting.

According to this embodiment, operating the wind turbine includes controlling the air deflectors as well as controlling the pitch angles of the wind turbine blades. These two options may both be applied in order to obtain a specified control objective, but their impact on noise generated by the wind turbine and/or on loads or wear on various wind turbine components may differ significantly. Thus, a suitable balance between activating the air deflectors and adjusting the pitch angles of the wind turbine blades may be selected, which ensures that the wind turbine operates within the noise constraints, while other overall control objectives are met.

The determined pitch angle setting could include a collective pitch angle as well as an individual pitch angle. A collective pitch angle is an angle which is applied to all of the wind turbine blades, and which is typically selected in order to match required power production, rotor/generator speed set-points and prevailing wind conditions. An individual pitch angle is a pitch angle which is applied individually to the wind turbine blades, typically superposed on the collective pitch angle. The individual pitch angle typically takes tilt/yaw loads and individual blade load considerations into account, such as tower passage, varying wind shear and/or yaw error depending on the position of the wind turbine blade in the rotor plane, etc.

Furthermore, the pitch angle setting may include settings regarding absolute pitch angles, pitch angle intervals, pitch angle rate of change, pitch angle rate of change limits, etc.

The step of determining a pitch angle setting may be performed while taking information regarding the determined air deflector setting into account, and the step of operating the wind turbine may include applying the determined air deflector setting and the determined pitch angle setting.

Air deflectors provide fast reaction times, but operating the wind turbine with activated air deflectors may result in an increased noise level being generated by the wind turbine. However, the load impact on the wind turbine blades, blade pitch bearings, etc. may be lower when applying air deflectors than when adjusting the pitch angles of the wind turbine blades. Accordingly, in the case that activating the air deflectors, in order to meet a specific control objective, will result in the noise generated by the wind turbine during operation exceeding the noise constraints, then it may be considered to de-activate at least some of the air deflectors, thereby reducing the generated noise to ensure that the wind turbine is operated within the noise constraints. However, this may have the consequence that the pitch angles of the wind turbine blades need to be adjusted to a greater extent in order to ensure that other control objectives are met. It is therefore an advantage that the step of determining a pitch angle setting is performed while taking information regarding the determined air deflector setting into account. Thereby the controller which performs the control of the pitch angles 'knows' how the air deflectors are controlled, or at least the state of the air deflectors and the expected impact with respect to the control objective. This knowledge is used for providing an appropriate control of the pitch angles. Thereby a synergy between the control of the air deflectors and the control of the pitch angles is obtained, which allows the control objective to be reached in an optimal manner, and while ensuring that the wind turbine is operated within the noise constraints.

The method may further comprise the step of obtaining load constraints for the wind turbine under the prevailing operating conditions, and the step of determining an air deflector setting may comprise determining an air deflector setting which enables the wind turbine to be operated within the obtained load constraints.

According to this embodiment, noise considerations as well as load considerations are taken into account when determining the air deflector setting. Furthermore, the determined air deflector setting enables the wind turbine to be operated within the obtained noise constraints as well as within the obtained load constraints. Thereby it is ensured that a reduction in the noise level generated by the wind turbine during operation is not obtained at the cost of subjecting the wind turbine to excessive loads.

The step of determining the air deflector setting may be based on historical or empirical data regarding wind turbine operation under various operating conditions, including various wind and weather conditions, such as wind speed, wind direction, wind shear conditions, gust conditions, precipitation, humidity, etc. Such historical or empirical data may include information regarding loads acting on various parts of the wind turbine, e.g. in the form of tilt/yaw loads, blade loads, tower loads, etc., under various operating conditions and at various air deflector settings. When certain operating conditions are prevailing, the historical or empirical data may provide a guide to selecting an air deflector setting which enables the wind turbine to operate within the obtained load constraints.

The step of determining an air deflector setting may comprise balancing noise considerations and load considerations. According to this embodiment, an air deflector setting is selected, which is optimal when taking noise considerations as well as load considerations into account. Thus, the determined air deflector setting may neither be the best option with respect to noise considerations, nor the best option with respect to load considerations. Instead, an air deflector setting is determined, which seeks to optimize noise generation as well as load impact. For instance, an air deflector setting which provides a small improvement with respect to noise generation should not be selected, if this results in a significant undesirable impact with respect to load impact, and vice versa.

The step of determining an air deflector setting may comprise determining an air deflector setting which optimizes power production of the wind turbine. According to this embodiment, the determined air deflector setting is the air deflector setting which enables the wind turbine to provide the highest possible power output, without risking that the noise constraints and/or the load constraints is/are exceeded.

The optimization of the power production of the wind turbine may be obtained by determining an air deflector setting and a pitch angle setting for the wind turbine, which enable the wind turbine to be operated within the obtained noise constraints as well as within the obtained load constraints. For instance, the optimization may be performed while taking information regarding a noise to lift ratio into account, e.g. information regarding noise to lift ratio of the individual air deflectors.

The step of determining an air deflector setting may comprise selecting an air deflector setting which temporarily allows the wind turbine to be operated outside the obtained noise constraints in order to enable the wind turbine to be operated within the determined load constraints.

In some cases, operating the wind turbine within the obtained noise constraints may have the consequence that the wind turbine, or one or more parts of the wind turbine, is subjected to excessive loads. When this is the case, it may be desirable to allow the noise constraints to be temporarily exceeded in order to avoid damage or overload to the wind turbine.

The step of operating the wind turbine may comprise adjusting a power output of the wind turbine and/or a rotational speed of the wind turbine in order to allow a given air deflector setting to cause the wind turbine to be operated within the obtained noise constraints.

According to this embodiment, in the case that it is not possible to enable the wind turbine to operate within the obtained noise constraints by controlling the air deflectors, possibly in combination with controlling the pitch angles of the wind turbine blades, the power output and/or a rotational speed of the wind turbine, e.g. the rotor speed, may be reduced in order to reduce the noise generated by the wind turbine to a level which is within the obtained noise constraints. For instance, reducing the rotor speed will reduce the relative speed between the wind and the wind turbine blade, thereby reducing the noise generated as a result of air deflectors being activated. Accordingly, reducing the rotor speed, and thereby the power production of the wind turbine, may allow activated air deflectors to be applied at a higher wind speed, without exceeding the noise constraints. This may be desirable for other purposes, e.g. in the case that the fast reaction time of the air deflectors is desired in order to handle fast changes in the wind conditions. However, since reducing the power output and/or the rotor speed results in a decreased production of the wind turbine, this will normally only be applied if it is not possible to meet the noise constraints in any other way or if other considerations, e.g. load considerations, are regarded as more important.

The step of obtaining noise constraints may comprise applying a model taking the prevailing operating conditions into account. The prevailing operating conditions could, e.g., include wind and/or weather conditions, such as wind speed, wind direction/yaw direction, precipitation, e.g. in the form of rain, sleet, snow, etc., the time of the day, the position of the wind turbine within a wind farm and/or with respect to noise sensitive neighbours, including distance to such neighbours, operation of other wind turbines arranged in the vicinity, e.g. forming part of a wind farm in which the wind turbine is arranged, including air deflector usage, power production and/or noise generation of such other wind turbines.

With respect to wind speed, the noise generated by the wind as such increases as the wind speed increases. Therefore, it may be acceptable that a wind turbine generates more noise at high wind speeds than at low wind speeds, because at higher wind speeds the noise generated by the wind turbine might be exceeded by the noise generated by the wind as such.

With respect to wind direction/yaw direction, a higher noise level generated by the wind turbine may be acceptable when the wind direction is such that the noise is carried away from the nearest and/or the most noise sensitive neighbours, than when the wind is in the opposite direction.

With respect to precipitation, some kinds of precipitation may generate a certain noise level. Similar to the remarks above with respect to the wind speed, an increased noise level generated by the wind turbine may therefore be acceptable in the case of certain kinds of precipitation and/or certain precipitation intensities.

With respect to the time of day, a higher noise level may be acceptable during the day than during the night.

With respect to the position of the wind turbine, a higher noise level may be acceptable if the distance to the nearest noise sensitive neighbours is large than when such neighbours are nearby. Furthermore, if the wind turbine is arranged well inside a wind farm, a higher noise level may be acceptable than if the wind turbine is arranged near the boundary of the wind farm. Furthermore, the noise generated by other wind turbines arranged nearby may have an impact on the acceptable noise level, since the total noise level must be maintained at an acceptable level.

As an alternative to obtaining the noise constraints by applying a model taking the prevailing operating conditions into account, the noise constraints may be obtained by consulting a look-up table, or in any other suitable manner.

The step of determining an air deflector setting may comprise selecting an air deflector setting from a predetermined set of air deflector settings. According to this embodiment, the air deflector setting is determined in a discrete manner, in the sense that only a limited number of air deflector settings are available.

The steps of obtaining noise constraints and determining an air deflector setting may be performed in a continuous manner. According to this embodiment, the air deflector setting is continuously adjusted to match continuously changing operating conditions.

The method further relates to a control system for controlling a wind turbine, the control system being adapted to perform the method described above, and to a wind turbine comprising a set of wind turbine blades, each wind turbine blade being provided with at least one air deflector being movable between an activated position in which it protrudes from a surface of the wind turbine blade and a de-activated position, the wind turbine further comprising such a control system.

Furthermore, the invention relates to a computer program product comprising program code which, when executed is adapted to perform the method described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 1 is a side view of a wind turbine blade being provided with a number of air deflectors and pressure sensors,

Fig. 2 is a cross sectional view of the wind turbine blade of Fig. 1,

Fig. 3 is a diagrammatic view of a control system for a wind turbine for performing a method according to an embodiment of the invention, and

Fig. 4 is a set of graphs illustrating a method according to an embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side view of a wind turbine blade 1 being provided with seven air deflectors 2 arranged on the suction side of the wind turbine blade 1. The air deflectors 2 are distributed along the length of the wind turbine blade 1. Each air deflector 2 is in the form of a plate which can be moved between an activated position and a de-activated position. In the activated position the air deflector 2 protrudes from the surface of the wind turbine blade 1. In the de-activated position the air deflector 2 is retracted to a position within the wind turbine blade 1.

Thus, when the air deflectors 2 are in the activated position, they disturb the air flow along the surface of the suction side of the wind turbine blade 1, thereby reducing the lift of the wind turbine blade 1.

The wind turbine blade 1 is further provided with seven pressure sensors 3 arranged on the suction side of the wind turbine blade 1, in such a manner that a pressure sensor 3 is arranged in the vicinity of each air deflector 2. Thereby local pressure measurements can be obtained at the positions of the each of the air deflectors 2. This allows the air deflectors 2 to be controlled on the basis of local pressure conditions.

Fig. 2 is a cross sectional view of the wind turbine blade 1 of Fig. 1. In Fig. 2 it can be seen that the wind turbine blade 1 is further provided with pressure sensors 3 arranged on the pressure side of the wind turbine blade 1, thereby allowing the air deflectors 2 to be controlled based on the local pressure conditions on the suction side, as well as on the pressure side, of the wind turbine blade 1. In the illustrated embodiment, the pressure sensor is shown to comprise two orifices at the blades surfaces and a central transducer. Arrow 4 indicates that the air deflector 2 can be moved between the activated position and the de-activated position.

Fig. 3 is a diagrammatic view of a control system 5 for a wind turbine for performing a method according to an embodiment of the invention. The control system 5 comprises a wind turbine controller WTC, 6, an air deflector controller ADC, 7 and a power plant controller PPC, 8. The wind turbine controller 6 controls the wind turbine as such, including controlling the pitch angles of the wind turbine blades 1. The air deflector controller 7 controls air deflectors mounted on the wind turbine blades 1. The power plant controller 8 controls a wind power plant or wind farm where the wind turbine is arranged. For instance, the power plant controller 8 may control the wind power plant or wind farm in order to ensure that the wind power plant or wind farm provides a required power output to a power grid. In an embodiment, the WTC 6 and the ADC 7 are implemented as functional units in a common control system.

In an embodiment, the air deflector controller 7 receives sensor input from the pressure sensors 3 arranged on the wind turbine blades 1. Based on these sensor input, the air deflector controller 7 controls the air deflectors 2, i.e. the air deflector controller 7 determines which of the air deflectors 2 should be in the activated state, and which should be in the de-activated state, when and for how long. Furthermore, the air deflector controller 7 activates the air deflectors 2 which need to be moved from the de-activated state to the activated state, and de-activates the air deflectors 2 which need to be moved from the activated state to the de-activated state. Finally, the air deflector controller 7 supplies information regarding the control of the air deflectors 2 to the wind turbine controller 6. The air deflector controller 7 may further provide the sensor measurements of the pressure sensors 3 directly to the wind turbine controller 6.

The air deflector controller 7 further receives noise constraints for the wind turbine from the power plant controller 8, via the wind turbine controller 6. The noise constraints depend on the prevailing operating conditions, as described above. Accordingly, the air deflector controller 7 further controls the air deflectors 2 in such a manner that the wind turbine is enabled to operate within the noise constraints. In addition, the air deflector controller 7 may receive load constraints from the wind turbine controller 6, and may thereby further control the air deflectors 2 in order to enable the wind turbine to operate within the load constraints.

Furthermore, the wind turbine controller 6 receives various sensor input and parameter input required in order to control the wind turbine appropriately. The wind turbine controller 6 also receives input from the power plant controller 8. This input could, e.g., include a power reference, grid information, etc. Based on the various input, the wind turbine controller 6 controls the wind turbine, including controlling the pitch angles of the wind turbine blades 1.

Fig. 4 is a set of graphs illustrating a method according to an embodiment of the invention. From top down the graphs illustrate noise generation by a wind turbine, activation of air deflectors, individual pitch amplitude and extreme flapwise loads, as a function of wind speed.

The dashed curves in the noise generation graph and the individual pitch amplitude graph represent a situation in which no air deflectors are activated. This could, e.g., represent operation of a wind turbine which is not provided with air deflectors. It can be seen that the noise generated by the wind turbine in this case is always below a noise limit. However, it is necessary to provide a relatively large individual pitch amplitude, in particular at higher wind speeds, in order to ensure that the flapwise loads are kept within an acceptable load limit.

The solid curves represent a situation in which the air deflectors are fully deployed at wind speeds above a certain threshold level, i.e. an air deflector setting is selected without taking noise considerations into account, but purely in order to meet other control objectives. It can be seen that this results in the noise generated by the wind turbine exceeding the noise limit for some wind speeds. However, the required individual pitch amplitude in order to ensure that the flapwise loads are kept within an acceptable load limit can be considerably reduced. This reduces the wear on the pitch system.

The dotted and dashed-dotted curves represent situations in which the air deflectors are activated, but the duty cycle of the air deflectors is reduced for some wind speeds in order to reduce the noise generated by the wind turbine to an acceptable level. The duty cycle could be reduced by de-activating some of the air deflectors and/or by controlling the air deflectors according to a pulse width modulation (PWM) control strategy, in which the air deflectors are activated and deactivated according to an appropriate switch pattern. In the case that the duty cycle is reduced by de-activating some of the air deflectors, the decision regarding which air deflector(s) to de-activate may be made based on a priori knowledge or an online estimate of lift to noise ratio of the individual air deflectors. The dashed-dotted curves represent a situation in which the duty cycle is reduced sufficiently to keep the noise generated by the wind turbine below the noise limit at all wind speeds. It can be seen that this has the consequence that the individual pitch angle amplitude must be larger than in the situation where the air deflectors are fully deployed, in order to ensure that the flapwise loads are kept within an acceptable limit. However, the individual pitch angle amplitude is still smaller than in the situation where no air deflectors are activated.

The dotted curves represent a situation in which the duty cycle is reduced as described above, but less than in the situation illustrated by the dashed-dotted curves. This has the consequence that the noise generated by the wind turbine exceeds the noise limit during a small wind speed range. However, this allows the individual pitch angle amplitude to be reduced as compared to the situation illustrated by the dashed-dotted curves during this wind speed range, without the flapwise loads exceeding the load limit. Thereby the wear on the pitch system is reduced, but this comes at the cost of a higher noise level.

Accordingly, the air deflectors and the individual pitch amplitude can be controlled in dependence of each other in order to obtain a desired noise level, a desired load impact and a desired level of wear on the pitch system. These considerations can be balanced according to a given situation, and an appropriate mix of air deflector activation and pitch control can be selected.

Claims (16)

1. A method for controlling a wind turbine, the wind turbine comprising a set of wind turbine blades (1), each wind turbine blade (1) being provided with at least one air deflector (2) being movable between an activated position in which it protrudes from a surface of the wind turbine blade (1) and a de-activated position, the method comprising the steps of: - obtaining noise constraints for the wind turbine under the prevailing operating conditions, determining an air deflector setting enabling the wind turbine to be operated within the obtained noise constraints, and operating the wind turbine while applying the determined air deflector setting.

2. A method according to claim 1, wherein the step of determining an air deflector setting comprises determining an air deflector setting which results in a specific control objective to be reached, unless the air deflector setting causes the wind turbine to operate outside the obtained noise constraints.

3. A method according to claim 1 or 2, further comprising the steps of: - determining a pitch angle setting for the wind turbine blades (1), the pitch angle setting enabling the wind turbine to be operated within the obtained noise constraints, and - operating the wind turbine while applying the determined pitch angle setting.

4. A method according to claim 3, wherein the step of determining a pitch angle setting is performed while taking information regarding the determined air deflector setting into account, and wherein the step of operating the wind turbine includes applying the determined air deflector setting and the determined pitch angle setting.

5. A method according to any of the preceding claims, further comprising the step of obtaining load constraints for the wind turbine under the prevailing operating conditions, and wherein the step of determining an air deflector setting comprises determining an air deflector setting which enables the wind turbine to be operated within the obtained load constraints.

6. A method according to claim 4 or 5, wherein the step of determining an air deflector setting comprising balancing noise considerations and load considerations.

7. A method according to any of claims 4-6, wherein the step of determining an air deflector setting comprises determining an air deflector setting which optimizes power production of the wind turbine.

8. A method according to claim 7, wherein the optimization of the power production of the wind turbine is obtained by determining an air deflector setting and a pitch angle setting for the wind turbine, which enable the wind turbine to be operated within the obtained noise constraints as well as within the obtained load constraints.

9. A method according to any of claims 5-8, wherein the step of determining an air deflector setting comprises selecting an air deflector setting which temporarily allows the wind turbine to be operated outside the obtained noise constraints in order to enable the wind turbine to be operated within the determined load constraints.

10. A method according to any of the preceding claims, wherein the step of operating the wind turbine comprises adjusting a power output of the wind turbine and/or a rotational speed of the wind turbine in order to allow a given air deflector setting to cause the wind turbine to be operated within the obtained noise constraints.

11. A method according to any of the preceding claims, wherein the step of obtaining noise constraints comprises applying a model taking the prevailing operating conditions into account.

12. A method according to any of the preceding claims, wherein the step of determining an air deflector setting comprises selecting an air deflector setting from a predetermined set of air deflector settings.

13. A method according to any of claims 1-11, wherein the steps of obtaining noise constraints and determining an air deflector setting are performed in a continuous manner.

14. A control system (5) for controlling a wind turbine, the control system (5) being adapted to perform the method of any of the preceding claims.

15. A wind turbine comprising a set of wind turbine blades (1), each wind turbine blade (1) being provided with at least one air deflector (2) being movable between an activated position in which it protrudes from a surface of the wind turbine blade (1) and a de-activated position, the wind turbine further comprising a control system (5) according to claim 14.

16. A computer program product comprising program code which, when executed is adapted to perform the method according to any of claims 1-13.