Claims:CLAIMS
We claim:
1. A method of controlling a wind turbine (10) by determining a pitch-offset-value (20), by pitching rotor blades (16), (17), (18) of the wind turbine (10) with respect to said pitch-offset value (20), whereby the pitch-offset-value (20) is determined based on the thrust on the wind turbine (10), in particular on a rotor (14) thereof, said method being characterized by the following steps:
a) determining or receiving a thrust-value (21) on the wind turbine (10), particularly on the rotor (14) thereof, as an input value into a data processing device;
b) based on the thrust-value (21), via said data processing device, determining a thrust-forecast-value (26), which is an estimated thrust-forecast-value (26) of the thrust-value (21);
c) based on the thrust-value (21), via said data processing device, determining a thrust-target value (32) as a reference value;
d) generating, via said data processing device, a resulting-thrust-value by bringing the thrust-forecast-value (26) in relation to the thrust-target-value (32);
e) from the resulting-thrust-value, via said data processing device, determining the pitch-offset-value (20) for at least one of the rotor blades (16), (17), (18).

2. The method according to claim 1, characterized in that the thrust-value (21) is obtained from at least one rotor blade signal of at least one rotor blade, preferably from rotor blade signals of all rotor blades, said rotor blade signal preferably being a rotor blade bending out-of-plane signal, and that the thrust-value (21) is derived from the at least one rotor blade signal.

3. The method according to claim 1 or 2, characterized in that the thrust-value (21) is obtained from at least one rotor blade bending out-of-plane moment of at least one rotor blade, preferably from the sum of the rotor blade bending out-of-plane moments of all rotor blades, and that the thrust-value (21) is derived from the at least one rotor blade bending out-of-plane moment (22), (23), (24).

4. The method according to anyone of claims 1 to 3, characterized in that the thrust-forecast-value (26) is determined by filtering and/or averaging the thrust-value (21), by determining, based on this signal, a slope (29) of the thrust-value (21) and by estimating the thrust-forecast-value (26) from the slope (29) of the thrust-value (21).

5. The method according to claim 4, characterized in that both, the filtered and/or averaged thrust-value (21) as well as the determined slope (29) of the thrust-value (21) are used to estimate the thrust-forecast-value (26).

6. The method according to anyone of claims 1 to 5, characterized in that the thrust-target value (32) is determined by averaging and/or filtering the thrust-value (21) over the time, in particular over a defined time period (34).

7. The method according to anyone of claims 1 to 6, characterized in that the thrust-target-value (32) is limited in the positive and/or negative direction.

8. The method according to anyone of claims 1 to 7, characterized in that the resulting-thrust-value is generated by calculating the difference between the thrust-forecast-value (26) and the thrust-target-value (32).

9. The method according to anyone of claims 1 to 8, characterized in that the pitch-offset-value (20) for the at least one of the rotor blades (16), (17), (18) is determined by multiplying the resulting-thrust-value with a gain-value (39).

10. The method according to anyone of claims 1 to 9, characterized in that for determining the pitch-offset-value (20) for the at least one of the rotor blades (16), (17), (18), the resulting-thrust-value, preferably the resulting-thrust-value multiplied with the gain-value (39), is limited in the positive and/or negative direction

11. The method according to anyone of claims 1 to 10, characterized in that for determining the pitch-offset-value (20) for the at least one of the rotor blades (16), (17), (18), the resulting-thrust-value, preferably the resulting-thrust-value multiplied with the gain-value (39), optionally limited in the positive and/or negative direction, is determined dependent on generator speed (45) of the wind turbine (10) and/or on wind speed (48).

12. A method of controlling a wind turbine (10) based on the thrust on the wind turbine (10), said wind turbine (10) comprising a tower (12) and a rotor (14) being mounted to the tower (12), said rotor (14) comprising a number of rotor blades (16), (17), (18), preferably three rotor blades, said method being characterized by the following steps:
a) determining or receiving a thrust-value (21) on the wind turbine (10), particularly on the rotor (14) thereof, as an input value into a data processing device;
b) determining, via said data processing device, a pitch-offset-value (20) for at least one of the rotor blades (16), (17), (18), preferably for all of the rotor blades, by making use of a method according to anyone of claims 1 to 11;
c) pitching at least one of the rotor blades (16), (17), (18), preferably all of the rotor blades, in accordance with the determined pitch-offset-value (20), in particular by pitching the rotor blade(s) out of the wind, if the thrust-forecast-value (26) is higher than the thrust-target-value (32), and vice versa.

13. A computer program product, which enables a data processing device, once the computer program product is executed on the data processing device, and is preferably stored in a storage device of the data processing device, to perform a method of determining a pitch-offset-value (20) according to anyone of claims 1 to 11, or to perform a method of controlling a wind turbine (10) according to claim 12.

14. A control device (19) of a wind turbine (10), said control device (19) being provided for controlling the wind turbine (10), in particular for controlling said wind turbine (10) based on the thrust on the wind turbine (10), particularly on the rotor (14) thereof, said control device (19) being provided in such a way that it is capable of executing the method according to anyone of claims 1 to 12, and/or that the control device (19) comprises a data processing device or an interface to an external data processing device, wherein a computer program product according to claim 13 is executed on said data processing device.

15. A wind turbine (10), said wind turbine (10) comprising a tower (12), a nacelle (11) being mounted to the tower (12), and a rotor (14) being mounted to the nacelle (11), said rotor (14) comprising a number of rotor blades (16), (17), (18), characterized in that the wind turbine (10) further comprises a control device (19) for controlling the wind turbine (10), in particular for controlling said wind turbine (10) based on the thrust on the wind turbine (10), said control device (19) being provided according to claim 14, or that the wind turbine (10) comprises means for performing the method according to anyone of claims 1 to 12.

Dated this 9th May 2019

Nandan Pendsey
(IN/PA/726)
AZB & Partners
(Authorized Agent of the Applicant)
, Description:METHOD OF CONTROLLING WIND TURBINE

FIELD OF THE INVENTION

The present invention generally relates to a wind turbine. More particularly, the present invention relates to a solution and a method of controlling a wind turbine depending on the thrust on the wind turbine. Further, the present invention relates to a method of determining a pitch-offset-value, said pitch-offset-value being used for controlling a wind turbine by pitching its rotor blades, whereby the pitch-offset value is determined based on and in dependence of the thrust on the wind turbine. Furthermore, the present invention is directed to a computer program product, to a control device of a wind turbine and to a wind turbine itself.

BACKGROUND

Wind turbines, in particular wind turbines of horizontal type, which means that the wind turbines comprise a horizontal rotor axis and a rotor being directed towards the wind, are known in the art.
Wind turbines generally comprise a nacelle incorporating a drive train. The nacelle is mounted to a tower. The rotor with a number of rotor blades, particularly with three rotor blades, is connected to the drive train via a hub, to which the rotor blades are mounted. The rotor rotates around its rotational rotor axis. According to one type of wind turbines the rotor blades are adjustably mounted to the hub. This is realized by means of respective pitch drives, said pitch drives being part of a pitch system. The pitch system, which is generally known in the prior-art, participates in the control of the rotor speed to given set points. In particular, the rotor speed is controlled by an electrical load of the generator device and by a pitch angle. By means of the pitch drives, the rotor blades can be moved about their rotor blade axes into different pitch positions.

In general, the amount of electric energy that can be generated with a wind turbine, is dependent on the size of the wind turbine. In recent years, wind turbines, in particular their towers and/or rotors, became therefore larger and larger. However, large towers and large rotors offer large surfaces to attack the wind, which results in high thrust forces for example. The costs of a tower and a foundation for the wind turbine are high due to high extreme and varying forces acting on. Therefore, the tower and the foundation have to resist the forces coming from the rotor of the wind turbine.

Recent developments in the technical field of wind turbines gave rise to new methods of mechanical-load-reducing control, that allow larger rotor diameters to be employed with less than proportional increases in material costs. One of such new developments is disclosed in US 9,631,606 B2. This known solution is directed to a system and to a method of dynamically controlling a wind turbine, wherein the control is performed in relation to the thrust on the wind turbine.

OBJECT OF THE INVENTION

Considering the aforementioned prior-art means, it is the object of the present invention to provide an easy to be carried out solution of reducing such forces acting on a wind turbine, such forces coming from the rotor of the wind turbine for example. In particular, it is the object of the present invention to provide a solution of reducing the magnitude and variation of such forces, in particular of thrust forces.

According to the present invention, the object is solved by the method with the features according to independent claim 1, which is the first aspect of the invention, by the method with the features of independent claim 12, which is the second aspect of the invention, by a computer program product with the features according to independent claim 13, which is the third aspect of the invention, by a control device with the features according to independent claim 14, which is the fourth aspect of the present invention, and further by a wind turbine with the features according to independent claim 15, which is the fifth aspect of the invention. Further, features and details of the invention become apparent from the dependent claims, from the description as well as from the drawings. Therein, features and details which are described in connection with one aspect according to the invention also apply with respect to this disclosure in their entirety to the other aspects according to the invention, so that any statements made with respect to one aspect of the invention also apply to their full extent to the other aspects of the invention, and vice versa.

The present invention is generally directed to a method of reducing the effects of forces acting on a wind turbine, in particular reducing the magnitude and the variation in the forces coming from a rotor of the wind turbine. In particular, the present invention is directed to a solution in the field of thrust control. In general, the thrust force, which is denoted as “thrust” in the following description, is a force acting on the wind turbine due to the wind and in the general direction of the wind. The present invention therefore provides a solution of reducing loads, in particular mechanical loads, which act on a tower and/or on the rotor of the wind turbine.

The underlying concept of the present invention includes that by suitably pitching the rotor blades of the rotor tower, fatigue and extreme loads can get reduced. In particular, according to the present invention, it can be ensured, that the thrust from the rotor is as constant as possible by pitching the rotor blades.

SUMMARY OF THE INVENTION

The present invention according to its five aspects is directed to the technical field of wind turbines, in particular to the technical field of horizontal wind turbines. Such wind turbines are generally known in the prior-art.

According to a preferred embodiment, a wind turbine comprises a nacelle which incorporates a drive train. The wind turbine comprises a rotor being connected to said drive train, said rotor preferably comprising a hub and a number of rotor blade, preferably three rotor blades, said rotor blades being mounted to said hub. The drive train transmits rotor speed to a generator device, where it is converted into electric energy. In particular, the drive train is the mechanical energy transmitting line starting from the hub of the rotor right up to the generator device. The rotor, which is directed towards the wind, is pivotally mounted - around a rotational rotor axis - to the drive train. In order to transform the rotational energy of the rotor into electric energy, the drive train, to which the rotor of the wind turbine is mounted, comprises a number of different components. One of these components is the generator device. The generator device generates electric energy. For this purpose, the generator device preferably comprises a stator component and a rotor component, said rotor component being coupled to a generator shaft. Preferably, the nacelle is mounted at the top of a tower of said wind turbine. At its lower base end, the tower is anchored to the ground by means of a foundation.

In particular, each rotor blade is adjustably mounted on the hub. This is realized by means of a pitch drive, said pitch drive being part of a pitch drive unit, which in turn is part of a pitch system. The pitch system, which is generally known in the prior-art, participates in the control of the rotor speed to given set points. In particular, the rotor speed is controlled by an electrical load of the generator device and by a pitch angle. By means of the pitch drives, the rotor blades may be moved about rotor blade axes into different pitch positions, said rotor blade axes extending in an axial direction of the rotor blade. The rotor blade having an adjustable pitch can be particularly understood in such a manner, that the angle of attack of the rotor blade, which may be defined as the pitch angle, can be adjusted or is provided in an adjustable manner, during a pitch-offset for example.

The present invention will now be explained in detail and with reference to the different aspects of the present invention.

It is the general principle of the present invention, that, based on at least one thrust-value resulting from the wind acting on the wind turbine, which is determined or received, different thrust-terms are determined, in particular mathematically created, calculated for example, and that these thrust-terms are set in relation to each other. Based on a result-value derived therefrom, at least one pitch-offset-value is generated, by means of which the rotor blades get pitched in relation to the wind.

According to the first aspect of the present invention, the object is solved by a method comprising the features of independent claim 1.

According to this first aspect, the present invention is directed to a method of controlling a wind turbine by determining a pitch-offset-value. “Determination” of said value preferably means, that the pitch-offset-value gets estimated, calculated or otherwise mathematically created, particularly in a data processing device. The pitch-offset-value is used for controlling the wind turbine by pitching at least one of its rotor blades with respect to said pitch-offset-value.

According to the present invention, the pitch-offset-value is determined based on the thrust on the wind turbine, in particular on the rotor thereof. As mentioned above, the thrust is a force, which is applied on a surface of a body, said force acting on the body in a direction perpendicular to or at least almost perpendicular to the surface of said body. In particular, according to the present invention, the thrust is the wind force acting on the wind turbine, in particular on the tower and/or on the rotor with its rotor blades.

The method according to the first aspect of the invention is characterized by the following steps:

In a first step a), a thrust-value on the wind turbine, particularly on the rotor thereof, is determined or received. This can be achieved in different ways. For example, the thrust-value is determined which means that the thrust-value is created - calculated or estimated for example - based on different parameters. In this case, the determination step of the thrust-value forms an integral part of the method according to the first aspect of the invention. According to a different embodiment, the thrust-value is received. In this case, the thrust-value may be an external value, which per se can be determined somewhere outside from the scope of the method. In this case, the thrust-value is measured, by means of suitable sensor elements for example, or determined, calculated or estimated for example, or both somewhere else. According to the latter embodiment, determining the thrust-value can or cannot form a part of the method according to the first aspect of the invention. Anyway, the thrust-value is used as an input value for the method according to the first aspect of the invention. In particular, the thrust-value is used as an input value which is entered into a data processing device. That means that the thrust-value is fed into the data processing device as an input value. In this case, the method according to the first aspect is preferably performed in such data processing device or by aid of such data processing device.

In general, the present invention is not limited to specific embodiments of how to determine the thrust-value. In the following, preferred embodiments are described in more detail further below.

According to one preferred embodiment, the thrust-value is obtained from at least one rotor blade signal of at least one rotor blade. The rotor blade signal preferably is a rotor blade bending out-of-plane signal. The thrust-value is derived from the at least one rotor blade signal.

Preferably, the rotor comprises a number of rotor blades, three rotor blades for example. In such a case, the thrust-value is preferably obtained from respective rotor blade signals of all rotor blades. Preferably, the rotor blade signals are added up.

In a preferred embodiment, the thrust-value is obtained from at least one rotor blade bending out-of-plane moment of at least one rotor blade, preferably from the sum of the rotor blade bending out-of-plane moments of all rotor blades, whereby the thrust-value is derived from the at least one rotor blade bending out-of-plane moment. Preferably, such rotor blade bending out-of-plane moments can get measured by means of suitable sensor elements for example. Or, they can get derived in different ways, calculated or estimated for example, from rotor and/or rotor blade parameters.

According to a preferred embodiment, the thrust-value is determined, calculated or estimated for example, as the sum of all, three for example in case of three rotor blades, rotor blade bending out-of-plane moments, which are known as flat moments as well. The thrust-value can be determined by measuring each flat – out of plane – moments in the rotor blades, in the blade roots for example.

When the thrust-value is determined from such moments as mentioned above, the thrust-value is derived from these moment value(s). It is assumed that there is a linear correlation between these moments and the thrust-value.

In a next step b), based on the thrust-value as derived in step a), via said data processing device, a thrust-forecast-value, which is an estimated thrust-forecast-value of the thrust-value, is determined. In general, a forecast is a statement of what is expected to happen in the future. The thrust-forecast-value therefore is a value indicating how the thrust on the wind turbine is expected to change in the future. Such a thrust-forecast-value can get determined in different ways. In the following, some preferred embodiments are described in more detail further below.

According to a preferred embodiment, the thrust-forecast-value is determined by filtering and/or averaging the thrust-value. The term “filtering” means that the thrust value, which was determined in step a), gets filtered by a suitable filter device, a low pass filter for example. Preferably, by means of such a filtering procedure, unwanted frequency content, high frequency content in case of a low pass filter for example, is filtered out. The term “averaging” means, that the thrust-value gets averaged by means of a suitable device of determining the average of the thrust-value. The filtering procedure and the averaging procedure can be used as equivalent procedures. In any case, a significantly smooth signal is ensured. Based on this filtered and/or averaged thrust-values, the slope of the thrust-value is determined. In general, the slope is the gradient of a graph at any point. The estimation of the slope is preferably executed by means of a suitable algorithm, which runs on the data processing device. Finally, the thrust-forecast-value is estimated from the slope of the thrust-value.

According to a preferred embodiment, both, the filtered and/or averaged thrust-value as well as the determined slope of the thrust-value are used to estimate the thrust-forecast-value. In this case, the slope together with the filtered and/or averaged thrust-value are used to estimate the forecast-thrust-value.

In a further step c), which can be performed subsequently to step b) or parallel to step b) or prior to step b), based on the thrust-value, via said data processing device, a thrust-target value is determined, said thrust-target-value being a reference value to the forecast-thrust-value.

According to a preferred embodiment, the thrust-target-value is determined by averaging the thrust-value over the time, in particular over a defined time period. For example, the thrust-value is averaged over a relatively long time, over 60 seconds for example, to obtain the thrust-target-value. Anyway, as described with regard to the forecast-thrust-value, it is also possible to determine the thrust-target-value using a filtering procedure, that is by filtering the thrust-value, by using low pass filtering for example.

To avoid the risk of ending up with an unwanted high or low thrust-target-value, it is preferably provided that the thrust-target-value is limited in the positive and/or negative direction. The limitation procedure is performed by means of a suitable limitation device or a limiter, which per se is known in the prior-art.

In a next step d), which follows on steps b) and c), via said data processing device, a resulting-thrust-value is generated by bringing the thrust-forecast-value in relation to the thrust-target-value. The resulting-thrust-value can be obtained in different ways.

According to a preferred embodiment, the resulting-thrust-value is generated by calculating the difference between the thrust-forecast-value and the thrust-target-value, in particular by subtracting the thrust-target-value from the thrust-forecast-value, which means the thrust-forecast-value minus the thrust-target-value. This difference and therefore the resulting-thrust-value can be denoted as the thrust-error. According to a preferred embodiment, it is provided to pitch the rotor blades out of the wind if the thrust-forecast-value is higher than the target-thrust-value, and vice versa.

In any case, in a final step e), a pitch-offset-value for at least one of the rotor blades is determined from the resulting-thrust-value via said data processing device. According to a preferred embodiment, respective pitch-offset-values for each rotor blade are determined.

According to a preferred embodiment, the pitch-offset-value for the at least one of the rotor blades is determined by multiplying the resulting-thrust-value with a gain-value.

Preferably, the gain-value is determined by means of a gain-calculation procedure, a gain-scheduling procedure for example. Such gain-calculation procedures per se are generally known in prior-art. In general, gain-scheduling is an approach for controlling non-linear systems by making use of linear mathematical models.

According to a preferred embodiment, a progressive gain is chosen, which means that the gain increases with the thrust error. Thus, a relative stronger pitch reaction can be obtained in case of a large thrust error.

According to a preferred embodiment, the resulting-thrust-value is multiplied with a gain to obtain the pitch-offset. Preferably, the gain-value is made linearly dependent on the thrust-error in order to build in, for example a progressive gain and thereby pitch action if needed.

For determining the pitch-offset-value for the at least one of the rotor blades, it is provided according to a preferred embodiment, that the resulting-thrust-value, preferably the resulting-thrust-value multiplied with the gain-value, is limited in the positive and/or negative direction. This limitation procedure is performed by means of a suitable limitation device or a limiter, which per se is known in the prior-art.

According to yet another preferred embodiment, for determining the pitch-offset-value for the at least one of the rotor blades, the resulting-thrust-value, preferably the resulting-thrust-value multiplied with the gain-value, optionally limited in the positive and/or negative direction, is determined dependent on the generator speed of the wind turbine and/or on the wind speed and/or on the pitch angle.

According to a preferred embodiment, the wind speed and/or the generator speed and/or the pitch angle and/or any other measures can be used or are used as input values for gain-scheduling of the pitch-offset-value.

Thus, a gain scheduling of the pitch offset is possible either using the wind speed and/or the generator speed and/or the pitch angle. Preferably, the pitch angle gets also averaged or filtered, low pass filtered for example, in a similar way as the wind speed and generator speed for gain scheduling of the pitch-offset-value. Gain scheduling using the pitch angle can be implemented in exactly the same way as the gain scheduling using wind speed and generator speed.

According to a preferred embodiment, as input signals for generating suitable pitch-offset-values for the rotor blades, and for the modulation of a common pitch-offset, three rotor blade bending out-of-plane signals, as well as the generator speed and/or the wind speed and/or the pitch angle can be used.

The pitch-offset-value that gets determined with the aforementioned method according to the first aspect of the invention, is preferably used in order to control the pitch of the rotor blades, in particular in order to influence and change the pitch angle of the rotor blades.

According to one preferred embodiment, which will be described next in connection with the second aspect of the invention, the pitch-offset-value is used for controlling the wind turbine in such a way.

According to the second aspect of the present invention, the object is solved by the method according to independent claim 12.

With regard to the configuration, to the performance and to the function of the method according to the second aspect, full reference is also made to the general description of the invention above, and to the description of the other aspects of the invention as disclosed above and further below.

According to the second aspect of the invention, a method of controlling a wind turbine based on the thrust on the wind turbine is provided. The wind turbine comprises a tower and a rotor being mounted to the tower, preferably on top thereof, said rotor comprising a number of rotor blades, preferably three rotor blades. The method is characterized by the following steps:

In a first step a) which can be, as described with regard to the first aspect of the invention, a part of the method according to the first aspect or a step being independent therefrom, a thrust-value on the wind turbine, particularly on the rotor thereof, is determined or received. The thrust-value is used as an input value into a data processing device, which means that the thrust-value is entered into the data processing device as input data.

In a next step b), via said data processing device, a pitch-offset-value for at least one of the rotor blades, preferably for all of the rotor blades, is determined by making use of the method according to the first aspect of the invention. Therefore, at this point, full reference is made to the entire disclosure of the first aspect of the invention.

In a next step c), at least one of the rotor blades, preferably all of the rotor blades, get pitched in accordance with the determined pitch-offset-value(s), in particular by pitching the rotor blade(s) out of the wind, if the thrust-forecast-value is higher than the thrust-target-value, and vice versa.

According to the third aspect of the present invention, the object is solved by the computer program product according to independent claim 13.

With regard to the configuration, to the performance and to the function of the computer program product according to the third aspect, full reference is also made to the general description of the invention above, and to the description of the other aspects of the invention as disclosed above and further below.

According to this third aspect, the invention is directed to a computer program product, which enables a data processing device, a data processing device as described above for example, once the computer program product is executed on the data processing device, and is preferably stored in a storage device of the data processing device, to perform a method of determining a pitch-offset-value according to the first aspect of the invention, or to perform a method of controlling a wind turbine according to the second aspect of the invention.

According to the fourth aspect of the invention, the object is solved by a control device comprising the features of independent claim 14.

With regard to the configuration, to the performance and to the function of the control device according to the fourth aspect, full reference is also made to the general description of the invention above, and to the description of the other aspects of the invention as disclosed above and further below.

According to this fourth aspect, the invention is directed to a control device of a wind turbine, said control device being provided for controlling the wind turbine, in particular for controlling said wind turbine based on the thrust on the wind turbine, particularly on the rotor thereof, said control device being provided in such a way that it is capable of executing the method according to the first and/or second aspect of the invention. Alternatively, or in addition the control device comprises a data processing device or an interface to an external data processing device, wherein a computer program product according to the third aspect of the invention is executed on said data processing device.

For example, the different components of the control device can be provided as electronic devices or as software components or as combinations of electronic devices and software components. The different components can be linked in or as a control loop. According to yet another preferred embodiment, the control device comprises a computer program product according to the third aspect of the invention, which is at least temporarily executed in or by the data processing unit of the control device. Preferably, the control device comprises means for performing the method according to the first and/or aspect of the invention.

According to the fifth aspect of the invention, the object is solved by a wind turbine, said wind turbine comprising the features of independent claim 15.

With regard to the configuration, to the performance and to the function of the wind turbine according to the fifth aspect, full reference is also made to the general description of the invention above, and to the description of the other aspects of the invention as disclosed above and further below.

According to the fifth aspect, the invention is directed to a wind turbine, said wind turbine comprising a tower, a nacelle being mounted to the tower, preferably on top of the tower, and a rotor being mounted to the nacelle, said rotor comprising a number of rotor blades. The wind turbine is characterized in that it further comprises a control device for controlling the wind turbine, in particular for controlling the wind turbine based on the thrust on the wind turbine, said control device being provided according to the fourth aspect of the invention. Alternatively, or in addition, the wind turbine comprises means for performing the method according to the first and/or second aspect of the invention.

It is an advantage of the invention according to its five aspects to significantly reduce the fore-aft tower fatigue and potentially extreme loads and thereby reduce costs of tower and foundation.

The present invention according to its five aspects provides, according to a preferred embodiment, a thrust control by observation and forecast and control of the thrust, preferably derived from rotor blade moments. By means of the present invention, undesired loads, such as tower fore/aft bending peaks can get reduced.

The invention will now be explained in more detail with respect to exemplary embodiments with reference to the enclosed drawings, wherein the foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures:

BRIEF DESCRIPTION OF THE INVENTION

Figure 1 shows a schematic view of a wind turbine incorporating the principles of the present invention; and
Figure 2 shows a control-block diagram of a method according to the present invention of determining a pitch-offset-value being used for controlling the wind turbine with regard to the thrust on the wind turbine.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 depicts a wind turbine (10) of the horizontal type with a tower (12) and a nacelle (11). Nacelle (11) is rotatable mounted to the top of tower (12). At its lower base end, the tower (12) is anchored to the ground by means of a foundation (13). Nacelle (11) incorporates a drive train (not shown), said drive train being mounted inside nacelle (11) and being connected to a rotor (14). Rotor (14) comprises three rotor blades (16), (17), (18) which are mounted to a hub (15). Hub (15) of rotor (14) is connected to a drive shaft of the drive train. The rotor blades (16), (17), (18) are adjustably mounted on the hub (15). This is realized by means of pitch drives (not shown), said pitch drives being part of a pitch system. The pitch system controls the rotor speed to given set points. By means of the pitch drives, the rotor blades (16), (17), (18) may be moved about rotor blade axes such that the pitch of the rotor blades (16), (17), (18) can be changed. This means that the pitch-angles of the rotor blades (16), (17), (18) can be changed such that the orientation of the rotor blades (16), (17), (18) can be varied.

The rotor (14) is rotatably connected to the drive train via its rotational axis. The drive train transmits the rotor speed to a generator device, where it is converted into electric energy. In order to transform the rotational energy of the rotor (14) into electric energy, the drive train may comprise the generator device (not shown).

Furthermore, a control device (19) is allocated to said wind turbine (10), said control device (19) being adapted of performing the method of determining a pitch-offset-value and of performing a method of controlling the wind turbine (10) based on such pitch-offset-values, said pitch-offset-values being used for controlling the wind turbine (10) by pitching its rotor blades (16), (17, (18) with respect to said pitch-offset-values, whereby the pitch-offset-values are determined based on and dependent on the thrust on the wind turbine (10), in particular on the rotor (14) thereof.

This method will now be described in detail in connection with Figure 2, which depicts a control-block diagram of a method of determining a pitch-offset-value (20) being used for controlling the wind turbine (10) with regard to the thrust on the wind turbine (10). This method is performed by making use of the control device (19). According to this embodiment, the pitch-offset-value (20) is generated.

When starting the method, in a first step, a thrust-value (21) is determined, said thrust-value (21) being obtained from the sum of the three rotor blades (16), (17), (18) bending out-of-plane moments (22), (23), (24) of the three rotor blades (16), (17), (18). In an inversion step (25), the resulting value gets inverted in order to provide the thrust-value (21), which is an input value for the method of determining the pitch-offset-value (20).

Next, in a second step, a thrust-forecast-value (26) is determined from the thrust-value (21). In order to obtain the forecast-thrust-value (26), the thrust-value (21) as determined in the first step gets filtered in a filtering step (27), low passed filtered for example, by using a time constant (28). Instead of such filtering step (27), an averaging step for averaging the thrust-value (21) can be performed as well. Next, based on the filtered/averaged signal, a slope (29) of the signal is determined, estimated for example, by making use of a time period (30) in the past. The filtered/averaged signal, which is the filtered/averaged thrust-signal (21), as well as the estimated slope (29) are used to estimate the thrust-forecast-value (26). Furthermore, a predicted time period (31) in the future is considered. At the end, the thrust-forecast-value (26) is available. The thrust-forecast-value (26) is an instantaneous value.

In a different step, a thrust-target-value (32) is generated. During an averaging step (33), the thrust-value (21) gets averaged over the time, by making use of a time period (34) in the past. Instead of such averaging step (33), a filtering step, a low pass filtering for example, can be performed as well. To avoid the risk of ending up with an unwanted high or the low thrust-target-value (32), it is preferably provided that the thrust-target-value (32) is limited in the positive and/or negative direction between an upper limit (36) and a lower limit (37). The limitation procedure is performed by means of a suitable limitation device (35).

During a subtraction step (38), a resulting-thrust-value which can be implemented as the thrust -error as well, is calculated as the thrust-forecast-value (26) minus the thrust-target-value (32). This signal is multiplied with a gain-value (39), which considers a gain slope (40) depending on the thrust and a gain offset (41), which is used to transform the thrust into a pitch-offset.

By means of another limitation device (42), the resulting pitch-offset-value is limited between an upper pitch-offset limit (43) and a lower pitch-offset limit (44). The resulting signal is provided as the pitch-offset-value (20).

Optionally, for determining the pitch-offset-value (20), generator speed (45) can be considered as a further input value. During a filtering/averaging step (46), the measured or estimated generator speed (45) gets filtered, low passed filtered for example, or averaged. By use of a look-up procedure (47), the resulting value can be correlated to and finally multiplied with the pitch-offset-value (20). As an alternative or in addition, for determining the pitch-offset-value (20), wind speed (48) can be considered as a further input value in the same way. During a filtering/averaging step (49), the measured or estimated wind speed (48) gets filtered, low passed filtered for example, or averaged. By use of a look-up procedure (50), the resulting value can be correlated to and finally multiplied with the pitch-offset-value (20). Moreover, a gain scheduling of the pitch-offset-value (20) is possible either using the wind speed (48) and/or the generator speed (45). Alternatively, or in addition, a pitch angle can be used as well. Preferably, the pitch angle gets also averaged or filtered, low pass filtered for example, in a similar way as the wind speed and the generator speed for gain scheduling of the pitch-offset-value.

The pitch-offset-value (20) is used in a method of controlling a wind turbine (10) based on the thrust on the wind turbine (10), said wind turbine (10) comprising a tower (12) and a rotor (14) being mounted to the tower (12), said rotor (14) comprising a number of rotor blades (16), (17), (18), preferably three rotor blades, said method being characterized by the following steps:
determining or receiving a thrust-value (21) on the wind turbine (10), particularly on the rotor (14) thereof, as an input value into a data processing device (not shown);
based on the thrust-value (21), via said data processing device, determining a thrust-forecast-value (26), which is an estimated thrust-forecast-value (26) of the thrust-value (21);
based on the thrust-value (21), via said data processing device, determining a thrust-target value (32) as a reference value;
generating, via said data processing device, a resulting-thrust-value by bringing the thrust-forecast-value (26) in relation to the thrust-target-value (32);
from the resulting-thrust-value, via said data processing device, determining the pitch-offset-value (20) for at least one of the rotor blades (16), (17), (18);
pitching at least one of the rotor blades (16), (17), (18), preferably all of the rotor blades, in accordance with the determined pitch-offset-value (20), in particular by pitching the rotor blade(s) out of the wind, if the thrust-forecast-value (26) is higher than the thrust-target-value (32), and vice versa.

The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles described herein can be applied to other embodiments without departing from the spirit or scope of the invention. Thus, it is to be understood that the description and drawings presented herein represent a presently preferred embodiment of the invention and are therefore representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art and that the scope of the present invention is accordingly not limited.

LIST OF REFERENCE NUMERALS

10 Wind turbine
11 Nacelle
12 Tower
13 Foundation
14 Rotor
15 Hub
16 Rotor blade
17 Rotor blade
18 Rotor blade
19 Control device
20 Pitch-offset-value
21 Thrust-value
22 Rotor blade bending out-of-plane moment
23 Rotor blade bending out-of-plane moment
24 Rotor blade bending out-of-plane moment
25 Inversion step
26 Thrust-forecast-value
27 Filtering/averaging step
28 Time constant
29 Slope
30 Time period
31 Predicted time period
32 Thrust-target-value
33 Averaging step
34 Time period
35 Limitation device
36 Upper limit
37 Lower limit
38 Subtraction step
39 Gain value
40 Gain slope
41 Gain offset
42 Limitation device
43 Upper pitch-offset limit
44 Lower pitch-offset limit
45 Generator speed
46 Filtering/averaging step
47 Look-up procedure
48 Wind speed
49 Filtering/averaging step
50 Look-up procedure.