WO2019001672A1

WO2019001672A1 - A method for controlling a wind turbine with blade profile changing means

Info

Publication number: WO2019001672A1
Application number: PCT/DK2018/050167
Authority: WO
Inventors: Jesper Sandberg Thomsen; Søren Hove SØRENSEN; Martin BRØDSGAARD; Ian Couchman
Original assignee: Vestas Wind Systems A/S
Priority date: 2017-06-27
Filing date: 2018-06-27
Publication date: 2019-01-03

Abstract

A method for controlling a wind turbine is disclosed. The wind turbine comprises a set of wind turbine blades (1), each wind turbine blade (1) being provided with profile changing means (2, 4) being configured to be in an activated state in which it changes an aerodynamic profile of the wind turbine blade (1) and in a de-activated state in which the aerodynamic profile of the wind turbine blade (1) is unchanged. The profile changing means could be in the form of air deflectors (2) or air outlets (4). Upon deciding activation of the profile changing means (2, 4), an expected impact, P, on power output of the wind turbine as a result of the decided activation of the profile changing means (2, 4), is estimated. A new reference power output, Pref,New, is provided, taking the estimated impact, P, into account. The profile changing means (2, 4) is activated, and the wind turbine is operated in accordance with Pref,New.

Description

A METHOD FOR CONTROLLING A WIND TURBINE WITH BLADE PROFILE CHANGING MEANS FIELD OF THE INVENTION

The present invention relates to a method for controlling a wind turbine comprising a set of wind turbine blades provided with profile changing means. More specifically, the invention relates to a method for controlling a power output 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 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.

Modern wind turbines are often controlled based on pitch actuation which regulates aerodynamic properties of the wind turbine blades by changing the angle of attack defined between the wind turbine blades and the incoming wind. Thus, by pitching the wind turbine blades, it is possible to control the lift and drag experienced by the wind turbine blades. It is normal practice to pitch the wind turbine blades according to some pitching strategy, in order to obtain a desired power production and load level for the wind turbine. To this end a reference power output may be applied.

The wind turbine blades of some wind turbines are further provided with profile changing means which, when activated, change the aerodynamic profile of the wind turbine blade, e.g. changing the lift and/or drag experienced by the wind turbine blade. Examples of such profile changing means include air deflectors and air outlets formed in the wind turbine blade.

Activating such profile changing means affects the power production of the wind turbine, and it will therefore affect the pitch control of the wind turbine, since the pitch control may attempt to compensate for the changes caused by the activated profile changing means.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a method for controlling a wind turbine, in which profile changing means can be applied without affecting a rotational speed of the wind turbine in an undesired manner. 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 profile changing means being configured to be in an activated state in which it changes an aerodynamic profile of the wind turbine blade and in a de-activated state in which the aerodynamic profile of the wind turbine blade is unchanged, the method comprising the steps of:

- operating the wind turbine in accordance with a first reference power output, P_ref,o, and with the profile changing means in the de-activated state,

- deciding activation of the profile changing means,

- estimating an impact, ΔΡ, on power output of the wind turbine as a result of the

decided activation of the profile changing means,

- providing a second reference power output, P_ref,New, the second reference power

OUtpUt, Pref,New, taking the estimated impact, ΔΡ, into account, and

- activating the profile changing means, and operating the wind turbine in accordance with the second reference power output, P_ref,New . 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 profile changing means being configured to be in an activated state and in a de-activated state. When in the activated state, the profile changing means changes an aerodynamic profile of the wind turbine blade. Accordingly, by activating the profile changing means of the wind turbine blade, the aerodynamic profile of the wind turbine blade can be deliberately changed, resulting in a change in the aerodynamic properties of the wind turbine blade, e.g. the lift and/or the drag. This change may take place much faster than changes caused by the pitching of the wind turbine blade. Accordingly, activation of the profile changing means may be applied in order to react to sudden changes in wind conditions, weather conditions, loads, etc. This will be described in further details below. When the profile changing means is in the de-activated state, the aerodynamic profile of the wind turbine blade remains unchanged, i.e. the aerodynamic profile of the wind turbine blade is the designed profile of the wind turbine blade.

According to the method of the invention, the wind turbine is initially operated in accordance with a first reference power output, P_ref,o, and with the profile changing means in the deactivated state. Accordingly, the aerodynamic profiles of the wind turbine blades are the original, designed aerodynamic profiles, and the wind turbine is operated as if the wind turbine blades were not provided with profile changing means.

The first reference power output, P_ref,o, is a target power production of the wind turbine. Accordingly, the wind turbine is controlled in such a manner that the power output of the wind turbine matches the first reference power output, P_ref,o . This is typically done by adjusting the pitch angles of the wind turbine blades in such a manner that a rotational speed of the rotor results in the desired power output.

Next, activation of the profile changing means is decided. This could, e.g., be in response to detecting that certain conditions regarding wind, weather, loads, or other relevant conditions are present. This will be described in further detail below. The profile changing means may be of a kind which can be fully or partially activated. For instance, the profile changing means may comprise two or more parts which can be individually activated and de-activated. In this case deciding activation of the profile changing means includes deciding to which extent the profile changing means is to be activated.

As described above, when the profile changing means is activated, the aerodynamic profiles of the wind turbine blades are changed. This results in a change in the aerodynamic properties of the wind turbine blades, and thereby a change in the ability of the wind turbine blades to extract energy from the incoming wind. Most often, the activation of the profile changing means will result in a degraded performance of the wind turbine blades.

Accordingly, if the wind turbine continues operating with the same pitch settings after activation of the profile changing means, the power output of the wind turbine will decrease. Therefore, in order to maintain the power output at the first reference power output, P_ref,o, the wind turbine controller will attempt to compensate for this by applying a more aggressive pitch setting. However, this results in an increase in the rotational speed of the rotor, and this is undesirable.

For instance, the rotor dynamics of a wind turbine where the profile changing means are deactivated may be regarded as: ]ώ_τ— τ_α τ₀ where J is the total inertia of the rotor system, ω_Γ is the rotational speed of the rotor, τ_α is the torque of the rotor, i.e. the input torque to the rotor system, _g is the torque of the generator, i.e. the output torque of the rotor system, and r_ioss is the torque loss introduced in the drive train. Thus, in order to obtain a given power output, corresponding to a given value of , while keeping the rotational speed of the rotor substantially constant, the input torque, τ_α, must be adjusted appropriately. As described above, this is normally achieved by appropriately adjusting the pitch angle of the wind turbine blades.

When the profile changing means are activated, the rotor dynamics are changed, and may be regarded as:

]ώ_Τ = τ_α— τ_Ρ where τ_Ρ is the torque change caused by the activation of the profile changing means.

Introducing τ_Ρ will initially decrease the rotational speed, but an active rotational speed controller will compensate for this by pitching in, and thereby increasing τ_α . The decreased pitch angle introduces a risk of maintaining or even increasing the structural loads.

In order to avoid this situation, an expected impact, ΔΡ, on power output of the wind turbine as a result of the decided activation of the profile changing means is estimated. Thus, it is estimated how much the power output of the wind turbine is expected to change, purely as a result of activating the profile changing means, and given that the current pitch setting is maintained. The estimation may be based on a model taking the original aerodynamic profile as well as the changed aerodynamic profile into consideration. Furthermore, the prevailing wind and weather conditions, in particular the prevailing wind speed, may be taken into consideration. Alternatively or additionally, the estimation may include calculations, consulting look-up tables, e.g. based on empirical data, etc. When the expected impact, ΔΡ, on power output has been estimated, a second reference power output, P_ref,New, is provided. The second reference power output, P_ref,New, takes the estimated impact, ΔΡ, on power output into account. For instance, the second reference power output, P_ref,New, may be calculated as

Pref,o-AP. Thus, the second reference power output, P_ref,New, represents a new reference value, or setpoint value, for the power output of the wind turbine, which takes into consideration that the aerodynamic properties of the wind turbine blades are changed when the profile changing means are activated.

Accordingly, the second reference power output, P_ref,New, can be reached by the wind turbine, after activation of the profile changing means, without resulting in undesired changes in rotational speed of the rotor of the wind turbine.

Finally, the profile changing means is activated, and the wind turbine is operated in accordance with the second reference power output, P_ref,New . Accordingly, the second power output, Pref,New, is applied as a new reference value, or setpoint value, for the power output of the wind turbine.

Thus, according to the method of the invention, when it is desired to activate the profile changing means, the expected impact thereof on the power production of the wind turbine is estimated, and a new reference value for the power output of the wind turbine is provided, taking the estimated impact into account. This ensures a smooth operation of the wind turbine, where the pitch control of the wind turbine blades will not attempt to compensate for the aerodynamic changes of the wind turbine blades caused by the activation of the profile changing means. Thereby an undesired decrease in pitch angle, and thereby increases in loads, can be avoided. However, activation of the profile changing means is an option, e.g. in order to handle loads on various wind turbine components, and/or in order to react fast to sudden changes in wind or weather conditions, e.g. in the form of gusts or sudden changes in wind speed or wind direction.

In the case that the expected impact of activation of the profile changing means is a degradation of the aerodynamic properties of the wind turbine blades, then the impact on the power output of the wind turbine must be expected to be negative. In this case, the second reference power output, P_ref,New, is essentially a derated power output reference. Accordingly, the wind turbine is, in this case, operated in a derated state after activation of the profile changing means. The step of estimating an impact, ΔΡ, on power output of the wind turbine may comprise determining coefficients of power, cp, relating to the unchanged aerodynamic profile and relating to the changed aerodynamic profile. Coefficients of power, cp, are specific for a given aerodynamic profile, and depend on the wind speed and the tip speed ratio of the wind turbine blade. For instance, a look-up table of Cp values relating to a given aerodynamic profile at various wind speeds and tip speed ratios may be provided, and this look-up table may be consulted when it is desired to obtain the current coefficient of power, cp, taking the prevailing wind speed and tip speed ratio into account. According to this embodiment of the invention, two such look-up tables may be provided, one relating to the original aerodynamic profile of the wind turbine blade, and one relating to the changed aerodynamic profile of the wind turbine blade, with the profile changing means in the activated state. When it is desired to activate the profile changing means, and it is therefore required to estimate the impact, ΔΡ, on the power output of the wind turbine, both look-up tables may be consulted, thereby obtaining a coefficient of power, cp, relating to the unchanged aerodynamic profile, and another coefficient of power, cp, relating to the changed aerodynamic profile, at the prevailing wind speed and tip speed ratio. Based on the obtained coefficients of power, cp, corresponding expected power outputs can be derived, and the difference between the derived expected power outputs provides a suitable estimate for the expected impact, ΔΡ.

The step of estimating an impact, ΔΡ, on power output of the wind turbine may comprise applying a dynamic model. The dynamic model may, e.g., include a low pass filter, such as a first order low pass filter. As an alternative, a more explicit dynamic model describing the activation and effect (torque and power) of the profile change, could be applied.

The wind turbine may be operated under full load conditions. At very low wind speeds, i.e. below a so-called cut-in wind speed, wind turbines do not produce power. When the cut-in wind speed is reached, the wind turbine starts operating, thereby producing power. However, at wind speeds between the cut-in wind speed and a nominal wind speed, the wind speed is insufficient to allow the wind turbine to produce a nominal or rated power production level. This is sometimes referred to as 'partial load conditions', and under these conditions the wind turbine is normally operated in such a manner that a maximum possible power output is obtained.

At wind speeds above the nominal wind speed, the nominal or rated power production level of the wind turbine can be reached. Therefore, under these conditions, the wind turbine is normally operated in such a manner that the power output of the wind turbine is limited to the nominal or rated power. This is sometimes referred to as 'full load conditions'.

Accordingly, under full load conditions, the first reference power output, P_ref,o, may be the nominal or rated power production level of the wind turbine. It is particularly advantageous to apply the method of the invention under full load conditions, because in this case the wind turbine is operated in such a manner that energy is not extracted from the wind to the maximum possible extent. Therefore, when the profile changing means are activated, it is possible for the pitch system to compensate for this and increase the power production of the wind turbine to the nominal or rated power production level. Thereby there is a high risk that the situation described above, where the pitch angle is decreased and the loads increase undesirably occurs.

Alternatively or additionally, the method of the invention may be applied under partial load conditions. The step of deciding activation of the profile changing means may be performed in response to a detected sudden change in wind conditions. Sudden changes in wind conditions could, e.g., include gusts, changes in wind speed, changes in wind direction, or any other suitable kind of sudden change. The sudden change in wind conditions may, e.g., result in increased loads on the wind turbine blades, increased tilt or yaw loads, increased nacelle acceleration levels, an increased rotational speed of the rotor, increased actuation of the pitch system, an increased rotor power, etc. Each of these impacts can be detected using a suitable kind of sensor. Alternatively or additionally, a given change in wind conditions may be detected directly, using a suitable kind of sensor, such as a wind speed sensor or a wind direction sensor. Alternatively or additionally, a sudden change in wind conditions may be detected based on statistical evaluations, such as a high standard deviation on any of the parameters mentioned above.

The profile changing means may comprise one or more air outlets formed in a surface of the wind turbine blade. According to this embodiment, the aerodynamic profile of the wind turbine blade is changed by ejecting air along a surface of the wind turbine blade, e.g.

towards the leading edge of the wind turbine blade, via the air outlet(s). This causes aerodynamic separation, thereby decreasing the lift and increasing the drag of the wind turbine blade.

The air flow may be provided in an active manner, e.g. using a pump or the like. As an alternative, the air flow may be provided in a passive manner, e.g. as a result of the rotational movements of the rotor, carrying the wind turbine blades. In any event, the wind turbine blades may be provided with one or more air channels arranged in the interior of the wind turbine blades, the air channels interconnecting the one or more air outlets and one or more inlet control valves. The inlet control valves may be arranged in a root part of the wind turbine blades, and the profile changing means may be activated by opening one or more of the inlet control valves, thereby causing air to be supplied to one or more of the air outlets, via the air channels. As an alternative, the profile changing means may comprise one or more air deflectors being configured to protrude from a surface of the wind turbine blade when the profile changing means is in the activated state.

In the present context the term 'air deflector' should be interpreted to mean a device being configured to extend from a surface, such as 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 changes the aerodynamic profile of the wind turbine blade, and thereby the aerodynamic properties of the wind turbine blade. For instance, the lift and/or the drag of the wind turbine blade may be affected. Thus, according to this embodiment, the change of the aerodynamic profile of the wind turbine blade is caused by a physical change of the surface of the wind turbine blade, due to the air deflector protruding from the surface.

The method may further comprise the steps of de-activating the profile changing means, and operating the wind turbine in accordance with the first reference power output, P_ref,o .

According to this embodiment, the operation of the wind turbine is returned to a normal operation mode when the profile changing means are once again de-activated. For instance, the profile changing means may be activated during a short period of time, e.g. while unusual wind conditions occur. In this case activation of the profile changing means may be initiated in order to handle loads occurring during such unusual wind conditions. When the unusual wind conditions are no longer occurring, the profile changing means may once again be de-activated, and the operation of the wind turbine returned to a normal operation mode.

The invention 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 profile changing means being configured to be in an activated state in which it changes an aerodynamic profile of the wind turbine blade and in a de-activated state in which the aerodynamic profile of the wind turbine blade is unchanged, 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 provided with profile changing means in the form of air deflectors,

Fig. 2 is a side view of a wind turbine blade provided with profile changing means in the form of air outlets, Fig. 3 is a cross sectional view of the wind turbine blade of Fig. 2,

Figs. 4-7 illustrate control of a wind turbine in accordance with a method according to an embodiment of the invention, and

Fig. 8 is a flow chart 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 profile changing means in the form of 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. Accordingly, the aerodynamic profile of the wind turbine blade 1 is changed.

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 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 side view of a wind turbine blade 1 being provided with profile changing means in the form of nine air outlets 4 formed in the suction side of the wind turbine blade 1. The air outlets 4 are distributed along the length of the wind turbine blade 1. Each air outlet 4 is arranged to be activated in such a manner that an air flow is ejected from the air outlet 4 and flows along the suction side of the wind turbine blade 1, towards the leading edge 5. This causes aerodynamic separation of the flow along the wind turbine blade 1, and thereby the aerodynamic profile of the wind turbine blade 1 is changed.

Fig. 3 is a cross sectional view of the wind turbine blade 1 of Fig. 2. In Fig. 3 it can be seen that the air outlets 4 are in the form of one way valves, allowing an air flow from the interior of the wind turbine blade 1 towards the surface of the wind turbine blade 1, via the air outlets 4, but preventing and air flow towards the interior of the wind turbine blade 1, via the air outlets 4.

Figs. 4-7 illustrate control of a wind turbine in accordance with a method according to an embodiment of the invention.

Fig. 4 shows activation and deactivation of profile changing means as a function of time. Figs. 5-7 illustrate simulations of pitch angle, generator speed and power output of the wind turbine, respectively, as a function of time during the activation of the profile changing means illustrated in Fig. 4. Figs. 5-7 show three different scenarios. The solid line 6 represents no compensation, i.e. that the wind turbine is controlled according to a prior art method during activation of the profile changing means. The dotted line 7 represents an ideal situation, in which the expected impact, ΔΡ, on power output of the wind turbine as a result of activation of the profile changing means has been accurately estimated and taken into account, and the system responds immediately to changes in the aerodynamic profile of the wind turbine blades. The dashed line 8 represents a situation in which the expected impact, ΔΡ, on power output has also been accurately estimated and taken into account, but where the system does not respond immediately to changes in the aerodynamic profile of the wind turbine blades. Thus, in the situation represented by the dashed line 8, the dynamics of the system have been taken into account, and it must therefore be expected that this scenario resembles a real situation more than the scenario represented by the dotted line 7.

It can be seen from Fig. 4 that the profile changing means are activated at time 50 s and deactivated at time 60 s. In the non-compensated scenario represented by the solid line 6 in Figs. 5-7, activation of the profile changing means has no effect on the control of the wind turbine. In particular, the power output remains at approximately 3300 kW, as it can be seen from Fig. 7, i.e. the reference power output of the wind turbine is not changed.

As can be seen from Fig. 5, the pitch system, in the non-compensated scenario, will attempt to compensate for the degraded aerodynamic properties of the wind turbine blades in order to maintain the power production. This leads to a significant change in pitch angle in response to the activation of the profile changing means at time 50 s as well as in response to de-activation of the profile changing means at time 60 s. As can be seen from Fig. 6, this in turn leads to significant changes in generator speed, i.e. the generator speed decreases significantly in response to activation of the profile changing means at time 50 s and increases significantly in response to de-activation of the profile changing means at time 60 s.

In the ideal scenario represented by the dotted line 7, the reference power output is decreased to 2800 kW in response to activation of the profile changing means at time 50 s. The decrease in the reference power output corresponds to an estimated impact on power output caused by the activation of the profile changing means. Since the dynamics of the system are not considered in this ideal scenario, the power production of the wind turbine immediately reaches the new, decreased reference power output. Similarly, the reference power output, and thereby the actual power production of the wind turbine, is increased to the original level of 3300 kW in response to the de-activation of the profile changing means at time 60 s. As can be seen from Figs. 5 and 6, in the ideal scenario represented by the dotted line 7 neither the pitch angle, nor the generator speed is affected by the activation and deactivation of the profile changing means. Accordingly, in this case the impact of the changed aerodynamic profile of the wind turbine blades is fully compensated by the decreased reference power output. In the scenario represented by the dashed line 8, the reference power output is also decreased to 2800 kW in response to activation of the profile changing means at time 50 s, and increased to the original 3300 kW in response to de-activation of the profile changing means at time 60 s, as can be seen from Fig. 7. However, in this case the dynamics of the system has the effect that the actual power output does not immediately reach the changed reference power output. Thereby a short transition time occurs following the time 50 s as well as the time 60 s, where the actual power production of the wind turbine differs slightly from the reference power output.

It can be seen from Figs. 5 and 6 that, in the scenario represented by the dashed line 8, the pitch angle as well as the generator speed are affected by the activation and de-activation of the profile changing means in a similar manner as was the case in the non-compensated scenario represented by the solid line 6. However, in this case the impact is much smaller than was the case in the non-compensated scenario. Accordingly, by providing a new, decreased reference power output when the profile changing means are activated, the wear on the pitch system is reduced, and undesired large changes in generator speed are avoided. Fig. 8 is a flow chart illustrating a method for controlling a wind turbine according to an embodiment of the invention. The process is started at step 9. At step 10 it is investigated whether or not a change in aerodynamic profile of the wind turbine blades is required. Such a change in aerodynamic profile of the wind turbine blades could, e.g., be required in the case that a sudden change in wind conditions has occurred, for instance a sudden change in wind direction and/or wind speed, or in the case of gusty wind conditions. Such a situation could be detected directly by means of a wind speed sensor or a wind direction sensor.

Alternatively, the situation would be detected in an indirect manner, e.g. by means of a suitable load sensor for measuring loads occurring at relevant positions and in relevant components of the wind turbine.

In the case that step 10 reveals that a change in the aerodynamic profile of the wind turbine blades is not required, the process is forwarded to step 11, where a standard reference power output, P_ref,o, is applied, and the wind turbine is operated with the profile changing means in the de-activated state, and in order to obtain a power output of the wind turbine which is equal to the reference power output, P_ref,o. Subsequently, the process is returned to step 10 for continued monitoring regarding the need for a change in the aerodynamic profile of the wind turbine blades.

In the case that step 10 reveals that a change in the aerodynamic profile of the wind turbine blades is required, the process is forwarded to step 12. At step 12, an expected impact, ΔΡ, on power output of the wind turbine as a result of activation of the profile changing means, is estimated.

At step 13, the profile changing means is activated, thereby changing the aerodynamic profile of the wind turbine blades. This could, e.g., include activating one or more air deflectors, thereby causing the air deflector(s) to protrude from a surface of each wind turbine blade. As an alternative, activation of the profile changing means may include activating an air flow out of one or more air outlets formed in a surface of each wind turbine blade, e.g. by opening one or more valves.

At step 14, a new reference power output, Pref,New Pref,0 -ΔΡ, is applied. Accordingly, the wind turbine is now operated with the profile changing means in the activated state, and in order to obtain a power output of the wind turbine which is equal to the new reference power

OUtpUt, Pref ,New .

At step 15 it is investigated whether or not the changed aerodynamic profile of the wind turbine blades is still required. If this is the case, the process is returned to step 14, and the wind turbine continues to operate with the profile changing means in the activated state, and in accordance with the new reference power output, P_ref,New .

In the case that step 15 reveals that the changed aerodynamic profile of the wind turbine blades is no longer required, the process is forwarded to step 16, where the profile changing means are de-activated, i.e. the original aerodynamic profile of the wind turbine blades is restored.

Subsequently, the process is returned to step 11, where the original reference power output, Pref,o, is once again applied.

Claims

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 profile changing means (2, 4) being configured to be in an activated state in which it changes an aerodynamic profile of the wind turbine blade ( 1) and in a de-activated state in which the aerodynamic profile of the wind turbine blade ( 1) is unchanged, the method comprising the steps of:

- operating the wind turbine in accordance with a first reference power output, P_ref,o, and with the profile changing means (2, 4) in the de-activated state,

- deciding activation of the profile changing means (2, 4), - estimating an impact, ΔΡ, on power output of the wind turbine as a result of the

decided activation of the profile changing means (2, 4),

OUtpUt, Pref,New, taking the estimated impact, ΔΡ, into account, and

- activating the profile changing means (2, 4), and operating the wind turbine in

accordance with the second reference power output, P_ref,New.

2. A method according to claim 1, wherein the step of estimating an impact, ΔΡ, on power output of the wind turbine comprises determining coefficients of power, cp, relating to the unchanged aerodynamic profile and relating to the changed aerodynamic profile.

3. A method according to claim 1 or 2, wherein the step of estimating an impact, ΔΡ, on power output of the wind turbine comprises applying a dynamic model.

4. A method according to any of the preceding claims, wherein the wind turbine is operated under full load conditions.

5. A method according to any of the preceding claims, wherein the step of deciding activation of the profile changing means (2, 4) is performed in response to a detected sudden change in wind conditions.

6. A method according to any of the preceding claims, wherein the profile changing means comprises one or more air outlets (4) formed in a surface of the wind turbine blade ( 1) .

7. A method according to any of claims 1-5, wherein the profile changing means comprises one or more air deflectors (2) being configured to protrude from a surface of the wind turbine blade (1) when the profile changing means (2) is in the activated state.

8. A method according to any of the preceding claims, further comprising the steps of de- activating the profile changing means (2, 4), and operating the wind turbine in accordance with the first reference power output, P_ref,o.

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

10. A wind turbine comprising a set of wind turbine blades (1), each wind turbine blade (1) being provided with profile changing means (2, 4) being configured to be in an activated state in which it changes an aerodynamic profile of the wind turbine blade (1) and in a deactivated state in which the aerodynamic profile of the wind turbine blade ( 1) is unchanged, the wind turbine further comprising a control system according to claim 9.

11. A computer program product comprising program code which, when executed is adapted to perform the method according to any of claims 1-8.