WO2021180282A1

WO2021180282A1 - Determination of wind field parameters of a wind turbine

Info

Publication number: WO2021180282A1
Application number: PCT/DK2021/050070
Authority: WO
Inventors: Thomas S. Bjertrup Nielsen; Søren DALSGAARD; Kim Hylling SØRENSEN
Original assignee: Vestas Wind Systems A/S
Priority date: 2020-03-09
Filing date: 2021-03-08
Publication date: 2021-09-16

Abstract

The invention relates to a method for determining a wind field inflow parameter based on monitoring parameters of a wind turbine with a variable rotor area, the wind turbine comprises a rotor with one or more rotor blades which are arranged hinged at a variable pivot angle, where the variable rotor area depends on the pivot angle, and where the pivot angle is dependent on a variable pivot force, the method comprises, determining a pivot angle signal which relates to the pivot angle of at least one of the rotor blades, and determining the wind field inflow parameter based on the pivot angle signal.

Description

DETERMINATION OF WIND FIELD PARAMETERS OF A WIND TURBINE

FIELD OF THE INVETION

The invention relates to a method for determining wind field parameters of wind turbines, particularly wind turbines having a rotor wherein blades are hinged to provide a variable rotor area.

BACKGROUND OF THE INVENTION

In order to provide efficient control of wind turbines, knowledge of wind field conditions such as turbulence and wind shear of individual wind turbines are essential parameters for improving control with respect to e.g. optimized power production and reduction of loads.

Accordingly, improved methods for determining wind field parameters for the control of wind turbines are needed.

A type of wind turbines are provided with wind turbine blades which are connected to a blade carrying structure via hinges which allows a pivot angle between the wind turbine blades and the blade carrying structure to be varied. Thereby, the diameter of the wind turbine rotor and consequently the rotor area can be varied.

The rotor area can be increased at low wind speeds to increase and optimize power production and decreased at high wind speeds where the wind energy may be sufficient for production of a nominal wind turbine power.

Due to the flexibility of the wind turbine to adapt to different wind speeds, improved and new methods for determining wind field parameters for the control of such variable-rotor-area wind turbines are also needed. SUMMARY

It is an object of the invention to improve determination of wind field inflow parameters such as mean wind speed, turbulence and wind shear of wind turbines. It is also an object of the invention to improve control of wind turbines based on improved or new methods for determining wind field inflow parameters. Particularly, it is an object to determine wind field inflow parameters of wind turbines having a variable rotor diameter and improving control of such wind turbines.

In a first aspect of the invention there is provided a method for determining a wind field inflow parameter based on monitoring parameters of a wind turbine with a variable rotor area, the wind turbine comprises a rotor with one or more rotor blades which are arranged hinged at a variable pivot angle, where the variable rotor area depends on the pivot angle, and where the pivot angle is dependent on a variable pivot force, the method comprises,

- determining a pivot angle signal which relates to the pivot angle of at least one of the rotor blades,

- determining the wind field inflow parameter based on the pivot angle signal.

Since the pivot angle of the blades are hinged at a variable pivot angle by means of a resilient blade hinge system the pivot angle is dependent on the variable pivot force, e.g. generated by an elastic member of the resilient blade hinge system, and the wind field inflow. In general, an equilibrium between the variable pivot force and the wind induced blade loads determines the resulting pivot angle. It is understood that the the pivot angle is dependent on a variable pivot force or equivalently on a variable pivot moment, which may be provided, at least partially, by pivot actuators such as force or moment actuators.

Advantageously, due to the dependency of the wind on the pivot angle signal which directly or indirectly correlates with the pivot angle, the pivot angle signal can be used for determining various wind field inflow parameter such as wind turbulence, means wind speed, wind shears and wind slopes.

According to an embodiment, the method comprises determining a standard deviation and/or a mean value of the pivot angle signal.

The mean value and consequently the standard deviation may be obtained over a given period of time, such as over periods ranging from seconds to years. The mean and standard deviation values may be based on values of the pivot angle signals obtained over the entire range of azimuth angles of the rotor or over specific subranges of 0-360 degrees azimuth range. According to an embodiment, the wind field inflow parameter comprises a wind turbulence parameter determined based on the standard deviation of the pivot angle signal and an obtained air density. Advantageously, the variation of the pivot angle signal provides information of the turbulence. Advantageously, the standard deviation may be obtained for different azimuth angles of the rotor so that turbulence can be determined for different zones within the rotor plane.

Similarly, in another embodiment, the wind field inflow parameter comprises a mean wind speed determined based on the mean value of the pivot angle signal and the air density.

According to an embodiment, the method comprises determining a mean pivot angle parameter difference based on mean pivot angle parameter values of the pivot angle signal determined for different azimuth angles of the one or more blades.

Advantageously, differences of the determined mean pivot angle parameter values of the pivot angle signal obtained for different azimuth angles or different subranges of different azimuth angles, can be used to determine wind field inflow parameters which generate different blade lift values at different azimuth angles, such as wind shear and wind slopes.

According to an embodiment, a first of the mean pivot angle parameter values is determined based on the pivot angle signal obtained when one of the blades points upwards and is parallel with or make an acute angle less than 45 degrees to a longitudinal direction of a tower of the wind turbine, and wherein a second of the mean pivot angle parameter values is determined based on the pivot angle signal 401 obtained when one of the blades points downwards and is parallel with or make an acute angle less than 45 degrees to the longitudinal direction of a tower of the wind turbine.

According to an embodiment, a third of the mean pivot angle parameter values is determined based on the pivot angle signal obtained when one of the blades points sideways and perpendicular with or makes an acute angle greater than 45 degrees to a longitudinal direction of a tower of the wind turbine, and wherein a fourth of the mean pivot angle parameter values is determined based on the pivot angle signal obtained when one of the blades points in the opposite sideways direction and is perpendicular with or makes an acute angle greater than 45 degrees to the longitudinal direction of a tower of the wind turbine.

According to an embodiment, the wind field inflow parameter comprises a vertical wind shear and/or a horizontal inflow of the wind determined based on the mean pivot angle parameter difference, such as the mean pivot angle parameter difference determined based on the first and second mean pivot angle parameter values.

Advantageously, the first and second mean pivot angle parameter values characterize differences in the blade lift at the upper and lower parts of the rotor plane, and can therefore be used for determining vertical wind shear and horizontal inflow.

According to an embodiment, the wind field inflow parameter comprises a horizontal wind shear and/or vertical inflow of the wind determined based on the mean pivot angle parameter difference, such as the mean pivot angle parameter difference determined based on the third and fourth mean pivot angle parameter values.

Advantageously, the third and fourth mean pivot angle parameter values characterize differences in the blade lift at the right and left parts of the rotor plane, and can therefore be used for determining horizontal wind shear and vertical inflow.

According to an embodiment, the method comprises determining a mean value of the wind field inflow parameter for different yaw angle intervals. Since the wind field inflow parameters may vary over different wind rose sections, it would be advantageous to determine the wind inflow parameter dependent on the different yaw angle intervals, e.g. to determine if wind inflow parameter deviates from historical values. According to an embodiment, the method comprises comparing the mean value of the wind field inflow parameter for one or more of the yaw angle intervals with historic values of a corresponding mean value of the wind field inflow parameter obtained for the same one or more angle intervals and determining a control parameter for the wind turbine or another wind turbine based on the comparison. Advantageously, the method may be used for detecting abnormal behavior by comparing wind field inflow parameters for a given rotor direction with historical values.

According to an embodiment, the method comprises controlling the wind turbine based on the determined wind field inflow parameter. Advantageously, the determined wind parameters can be used for controlling the wind turbine, e.g. to avoid or limit loads on the blades and tower and to optimize power production of a given wind turbine or a wind turbine park.

According to an embodiment, controlling the wind turbine comprises adjusting the pivot angle based on the determined wind field inflow parameter. Advantageously, the pivot angle can be adjusted in order to adjust power production and rotor plane loads.

A second aspect of the invention relates to a wind field estimator arranged to perform the steps according to the first aspect.

A third aspect of the invention relates to a wind turbine comprising a rotor with a variable rotor area, where the rotor comprises one or more rotor blades which are arranged hinged at an adjustable pivot angle, where the variable rotor area depends on the pivot angle, and where the pivot angle is adjustable dependent on a variable pivot force provided by a pivot actuator, and the wind field estimator according to the second aspect.

In general, the various aspects and embodiments of the invention may be combined and coupled in any way possible within the scope of the invention.

These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which Figs. 1 and 2 show a wind turbine comprising hinged rotor blades,

Fig. 3 shows a detailed view of a blade hinged to the arm of the blade carrying structure of the rotor,

Figs. 4A and 4B show a wind field inflow estimator,

Fig. 5A shows an example of the relationship between wind speeds and pivot angles,

Fig. 5B shows an example of the relation between the standard deviation of the pivot angles and the wind speed, and

Fig. 6 shows the rotor plane of a wind turbine with indication of different azimuth positions.

DESCRIPTION OF EMBODIMENTS

Figures 1 and 2 show an example of a wind turbine 100 (WTG) comprising a tower 101 and a rotor 102 with at least one rotor blade 103, such as three blades. Fig. 1 shows a front view with the blades facing the wind and Fig. 2 shows a side view seen perpendicular to the wind direction 110. The blades 103 are connected with the hub 105 which is arranged to rotate with the blades. The hub 105 comprises a blade carrying structure 106 which may be configured as a structure with arms, one per blade, extending radially relative to the main shaft axis of the hub to end- portions of the arms. The rotation axis of the main shaft axis is indicated with reference 111. The blades 103 are connected to the blade carrying structure 106, such as the arms thereof, via a hinge 108.

The rotor is connected to a nacelle 104 which is mounted on top of the tower 101 and is adapted to drive a generator situated inside the nacelle via a drive train comprising the main shaft axis 111. The rotor 102 is rotatable by action of the wind. The wind induced rotational energy of the rotor blades 103 is transferred via a shaft to the generator. Thus, the wind turbine 100 is capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades and, subsequently, into electric power by means of the generator. The generator is connected with a power converter, such as a power converter configured with a generator side converter and a line side converter where the generator side converter converts the generator AC power into DC power and the grid side converter converts the DC power into an AC power for injection into the power grid. The generator and the power converter is part of the power generating system of the wind turbine.

The wind turbine 100 is configured so that in a normal power producing operation, the rotor 102 is arranged on the lee side of the tower 101, i.e. as illustrated with the wind direction 110, the rotor is located to the right of the tower 101.

The blades may be hinged at a location between an outer blade tip 113 and an inner blade tip 114 so that the blade 103 comprises an inner blade portion 103a extending between the hinge location and the inner blade tip 114 and an outer blade portion 103b extending between the hinge location and the outer blade tip 113. During normal operation, the inner blade portion 103a extends from the hinge location towards the main shaft axis and the outer blade portion 103b extends outwards away from the main shaft axis, at least for a range of pivot angles. As is seen in Fig. 3, the inner blade portion 103a extends towards the main shaft axis 111 for pivot angles from 0 to 80 degrees, assuming that the acute angle between the longitudinal extensions of the inner and outer blade portions is 10 degrees. At the 90 degrees pivot angle, the inner blade portion 103a points away from the main shaft axis 111.

Due to the hinged connection, the wind turbine blades 103 are able to perform pivot movement relative to the blade carrying structure 106. The pivot angle a is defined as the angle between the longitudinal axis of the outer blade portion 103b axis and a plane normal to the main shaft axis. A pivot angle of 0 degrees means that the outer blade is normal to the main shaft axis and maximal rotor area occurs at this angle.

The rotor area is defined as the area within the outer blade tips 113 in a plane perpendicular to the main shaft axis. The actual swept area swept by the rotor blades is the area between the inner and outer blades tips 113, 114 in a plane perpendicular to the main shaft axis. The rotor area varies as a function of pivot angle in such a manner that the rotor area is at a maximum when the pivot angle is at a minimum, and at a minimum when the pivot angle is at a maximum.

Fig. 3 shows a more detailed view of one arm of the blade carrying structure 106 with the blade 103 hinged to the arm.

The rotor 102 is designed to carry blade loads through the pivot hinge 108 and the pivot actuator 301 to the arm structure 106. This design allows the blades 103 to pivot around the hinge axis.

As illustrated, the pivot actuator 301 may be hydraulic actuator such as a hydraulic cylinder. For example as illustrated in Fig. 3, the position of the piston in the hydraulic cylinder is mechanically connected with the inner blade portion 103a, e.g. near the inner blade tip 114. The pivot actuator or the pivot system comprising the pivot actuator and the mechanical connection may be configured as an elastic or flexible connection so that the resulting pivot angle a is partly determined by the pivot force/moment provided by the pivot actuator 301 and partly by the force acting on the rotor blade 103 such as the aerodynamic forces. The mechanical connection may comprise an elastic member 302 such as a spring. Alternatively, the elastic property of the pivot actuator may be achieved by controlling the position of the piston dependent on a measured piston force, e.g. so that the position is controlled according to Hookes law.

Fig. 3 illustrates the orientation of the outer blade portion 103a for different wind levels.

The pivot angle a can be adjusted by a variable pivot force F or variable pivot moment M provided by a pivot actuator 301 or by other means. Thus, the pivot actuator 301 may be a force actuator arranged to generate a displacement via an applied force, or may be a torque actuator arranged to generate an angular displacement via an applied torque. The pivot actuator 301 may be configured to be able to generate a desired pivot force F or pivot moment M. For example, the pivot actuator may comprise a feed-back control system arranged to control the pivot actuator to generate the desired pivot force or pivot moment. Herein, the pivot force F and pivot moment M are equivalent and the pivot actuator may be configured to provide a desired force or equivalently a desired moment. The relationship between the pivot force and the pivot moment is given by the distance between the hinge where the moment acts or is applied and a location on the inner blade portion 103a where the pivot force acts or is applied. The pivot actuator 301 configured as a moment actuator is arranged in the hinge of the hinged blades. Such hinge actuator is particularly relevant when the blades are hinged close to the hub 105. An alternative moment actuator is embodied by movable mass located inside the blade where the moment is generated from the centrifugal forces and can be adjusted by moving the mass in or out along the blade.

As will be clear from the description, adjusting the pivot angle a by use of the pivot actuator does not necessarily mean that the pivot angle a is controlled to approach a desired pivot angle. Adjusting the pivot force merely means that the actual pivot angle can be affected by the pivot force, but where the resulting pivot angle depends on a force equilibrium between the pivot actuator force generated by the pivot actuator 301, a wind load force generated due to the rotor thrust and elastic properties of the pivot actuator. This is particularly the case when the pivot system or pivot actuator is configured with an elastic/flexible connection.

The rotor thrust is the load on the rotor 102 generated by the incoming wind and dependent on the aerodynamic properties of the blades 103.

Thus, in general the resulting pivot angle is obtained dependent on a balance between at least the generated pivot force and a wind load force generated in response to the wind load on the rotor 102. Other forces generated due to the elastic properties of the pivot actuator, centrifugal forces and/or aerodynamic forces are also included in the equilibrium and thereby affects the resulting pivot angle a.

For example, with a given set-point for the actuator force, the force equilibrium implies that an increased wind speed and thereby increased wind thrust leads to an increase of the pivot angle a. This has the advantage that the rotor area may decrease in response to a wind gust. Additionally, centrifugal forces and/or aerodynamic forces acting on the wind turbine blades 103 cause the wind turbine blades to pivot towards larger pivot angles a for increasing wind speeds. Accordingly, at any given wind speed, the wind turbine blades will find an equilibrium pivot angle which balances the various forces acting on the wind turbine blades. The higher the wind speed, the larger the equilibrium pivot angle will be.

Due to the relation between the wind speed and the resulting pivot angle a, or generally, the relation between the or aerodynamic forces acting on the wind turbine blades 103 and the resulting pivot angle a, the inventors has realised that it is possible to determine a wind field inflow parameter 402 based a determined pivot angle signal 401 which relates to the pivot angle of at least one of the rotor blades 103.

The pivot angle signal 401 may be based on measuring the pivot angle of at least one of the rotor blades. For example, the pivot angle signal 401 may be determined as the measured pivot angle, e.g. measured by a linear or rotational encoder arranged with the pivot system, or a parameter derived from the measured pivot angle such as a low pass filtered signal of the measured pivot angle, a standard deviation of the pivot angle, or a mean value of the pivot angle.

Alternatively or additionally, the pivot angle signal 401 may be determined based on other variables of the pivot system which are related, i.e. correlated, with the pivot angle a. For example, the pivot angle signal may be determined based on a pressure of a hydraulic tank of the hydraulic pivot actuator 301, a flow of oil of a hydraulic pivot actuator 301, a current supplied to drive an electric pivot actuator 301. Such physical values which can be measured or estimated strongly correlates with the pivot angle a and can therefore be used for determining the wind field inflow parameter 402.

Also in this case where the pivot angle signal 401 is determined based on values which correlate with the pivot angle a, statistically derived signals such as a low pass filtered signal, a standard deviation, a mean value, or other, may be determined from the pivot angle signal 401. The pivot angle signal 401 may include values such as sampled values of the measured pivot angle a or the correlating signal.

Examples of the wind field inflow parameter 402 which can be determined based on the pivot angle signal 401 includes mean wind speed, wind turbulence, wind shear, wind inflow angles as described in further detail below.

The mean value of the pivot angle or the pivot angle signal 401 may be determined as the mean value of relevant values within a time window such as a rolling time window. For example, the rolling time window may have a given length, e.g. a 1 min. length, and new data is included in the data set of the window at every sample, e.g. once every 0.1 s, while the oldest data in the data set is discarded. For example, the mean value may be determined according to sum_i(ai)/N, where ai is a sample of the pivot angle or pivot angle signal 401 of one blade, or possibly the average over all blades per sample, where N is the number of samples, and where the sum is over the time window period.

The standard deviation of the pivot angle or pivot angle signal 401 may be determined similarly based on a data set of pivot angles, e.g. a set of samples, of the pivot angle signal 401 within a time window, e.g. according to the equation sqrt{sum_i(ai - am)/N}, where am is the mean value of the pivot angle or the pivot angle signal 401, e.g. determined as described above.

Fig. 4A shows a wind field estimator 400 for determining the wind field inflow parameter 402 based on the pivot angle signal 401 and optionally other input parameters 403 such as power production of the wind turbine 100 and air density.

In one example, the wind field inflow parameter is a wind turbulence parameter which is determined based on the standard deviation of the pivot angle signal and air density.

On wind turbine sites with high turbulence levels and during operation with a certain pivot force the pivot angle variations will be larger compared to sites with lower turbulence levels. Further, the variations in power and rotor speed will be different. Accordingly, the standard deviation of the pivot angle a or pivot angle signal 401, or other calculations reflecting the variations of the pivot angle or pivot angle signal, can be used to determine the wind turbulence parameter. The relationship between the turbulence and the variations of the pivot angle further depends on the air density in the way that higher air densities result in higher turbulence values for the same variations of the pivot angle. Accordingly, wind turbulence parameters may be determined based on both the variation of the pivot angle and the air density. The air density can be determined based on air pressure and temperature measurements according to known methods.

Fig. 4B shows an example of the wind field estimator 400 for determining the wind turbulence parameter as comprised by the wind field inflow parameter 402.

In this example, the wind field estimator 400 comprises a filter 411 arranged for removing frequency content of the pivot angle signal 401 which could lead to incorrect or less accurate determinations of the turbulence. For example, the frequency content at the lP-frequency (i.e. the rotational frequency of the rotor) would merely indicate shear and yaw error, but not turbulence. Frequency content at the tower resonance frequency could also affect the determination of turbulence. Accordingly, the determination of the wind turbulence parameter may include a step of filtering the pivot angle signal 401 in order to remove frequency content in certain spectral ranges.

The determination of the standard deviation of the pivot angle signal 401, or of the filtered pivot angle signal 401, is carried out by a standard deviation calculation block (not shown) of the wind field estimator 400.

The wind field estimator 400, according to this example, further comprises a functional relationship 412 such as a look-up table between the standard deviation of the pivot angle signal 401, or standard deviation of the filtered pivot angle signal 401, optionally the air density provided via input 403 and the estimated wind turbulence parameter of the wind field inflow parameter 402.

The mean value of the pivot angle a or pivot angle signal 401, or other calculation reflecting the average of the pivot angle signal e.g. over a running time window, can be used to determine the mean wind speed. Similarly to the determination of the wind turbulence parameter, the relationship between the mean value of the pivot angle signal 401 and mean wind speed depends on the air density. In this case, the frequency content in certain spectral ranges may also inappropriately affect the determined mean wind speed why filtering may be used for removing such frequency content.

Accordingly, the wind field estimator 400 of Fig. 4B can be used for determining the mean wind speed based on the air density provided via input 403. In this case the wind field estimator 400 comprises a mean value calculation block (not shown) configured to determine the mean value of the pivot angle signal 401, or the filtered pivot angle signal. The wind field estimator 400 also comprises a functional relationship or look up table (similar to the block 412) between the mean value of the pivot angle signal 401, or the mean value of the filtered pivot angle signal 401, the air density provided via input 403 and the estimated mean wind speed of the wind field inflow parameter 402.

Fig. 5A shows an example of the functional relationship 412 between wind speeds and pivot angles, or mean values thereof, for determining the mean wind speed. Two relations are presented, the dashed line representing higher air density then the solid line. If the pivot angle, or the mean value of the pivot angle signal 401, and the air density are known, the mean wind speed can be found using this relationship.

The slope of the curves are relatively low in low wind speeds. Therefore, a relative small error in the pivot angle signal 401, e.g. due to an estimation error, will cause a larger error in the estimate of the wind speed as compared to higher wind speeds. This sensitivity toward errors in the pivot angle signal 401 can be compensated by including power production as an additional source of information. The power production may be provided via input 403. That is, the power production P may be given by P = V2 p A Cp n^L3, with p = air density A = rotor area, Cp = aerodynamic efficiency and V = wind speed. For low wind speeds the pivot angle and consequently the rotor area A changes only little. Assuming that Cp is kept at optimal then the wind speed V is proportional to the R^L(1/3) as an approximation with A substantially constant, or proportional to (R/A)^L(1/3). Thus, inclusion of the power P may provide a better estimate of the wind speed V compared to using only the pivot angle a.

Fig. 5B shows an example of the relation between the standard deviation of the pivot angles and the wind speed. The two curves including the dashed-line curve obtained for high turbulent wind and the solid-line curve obtained for low turbulent wind show the correlation between the standard deviation of the pivot angle and level of wind turbulence. The peak of the curves is due to the fact that the coupling between changes in the wind speed and changes in pivot angles are maximal in the range of wind speeds of the peaks.

Accordingly, the functional relationship 412 between standard deviation of the pivot angle signal 401 and the wind turbulence parameter can be obtained based on turbulence values for different wind speeds and the standard deviation of the pivot angle. In this relationship, the wind speed can be obtained from the pivot angle signal as described in connection with Fig. 5A, so that the wind turbulence parameter can be obtained from the pivot angle signal 401 and the standard variation thereof.

Fig. 6 shows the rotor plane of a wind turbine 100 with indication of different azimuth positions. Fig. 6 and the following explanation shows how the pivot angle signal 401 can also be used for determining vertical and horizontal wind shear and horizontal and vertical inflow.

Vertical wind shear is defined as vertical (i.e. along the longitudinal direction 601 of the tower) variation of the wind speed over the rotor area. The different wind speeds at the upper and lower portions of the rotor plane results in that the angle-of-attacks and lift at the rotor 102 are different for different azimuth angles of the one or more blades 103. Particularly, for azimuth angles of the one or more rotor blades 103 corresponding to locations of a rotor blade 103 near the 6 o'clock and 12 o'clock positions, different lift values at the two locations results in different pivot angles a due to different wind induced blade loads.

Horizontal inflow is defined as the angle in a horizontal plane (perpendicular to the longitudinal direction 601 of the tower) between the incoming wind (wind vector) and a vector perpendicular on the rotor plane. Thus, the horizontal inflow defines a difference between the direction of the wind and the normal to the rotor plane, in the horizontal plane. Due to the angle of the wind relative to the normal of the rotor plane, the angle-of-attacks and lift at the rotors 102 are different for different azimuth angles of the one or more blades similarly to vertical wind shear. Thus, for azimuth angles of the one or more rotor blades 103 corresponding to locations of a rotor blade 103 near the 6 o'clock and 12 o'clock positions, a difference of the pivot angles a are generated due to different lift values generated by the horizontal inflow.

The vertical wind shear and the horizontal inflow of the wind may be determined based on a mean pivot angle parameter difference Aa_m determined based on first and second mean parameter values al_m, a2_m, e.g. as Aa_m = al_m- a2_m.

The first mean pivot angle parameter value al_m is determined based on the pivot angle signal 401 obtained when one of the blades 103 points upwards (e.g. near the 12 o'clock position) and is parallel with or make an acute angle less than 45 degrees to a longitudinal direction 601 of a tower 101. The second mean pivot angle parameter value a2_m is determined based on the pivot angle signal 401 obtained when one of the blades points downwards (e.g. near the 6 o'clock position) and is parallel with or make an acute angle less than 45 degrees to the longitudinal direction 601 of a tower 101.

Horizontal wind shear is defined as horizontal (i.e. perpendicular to the longitudinal direction 601 of the tower) variation of the wind speed over the rotor area. The different wind speeds at the left and right portions of the rotor plane results in that the angle-of-attacks and lift at the rotors 102 are different for different azimuth angles of the one or more blades. Particularly, for azimuth angles of the one or more rotor blades 103 corresponding to locations of a rotor blade 103 near the 3 o'clock and 9 o'clock positions, different lift values at the two locations results in different pivot angles a due to different wind induced blade loads. Vertical inflow, also known as terrain slope, is defined as the angle in a vertical plane (parallel to the longitudinal direction 601 of the tower) between the incoming wind (wind vector) and a horizontal plane perpendicular to the longitudinal direction 601. Thus, the vertical inflow defines a difference between the direction of the wind and the normal to the rotor plane, in the vertical plane. Due to the angle of the wind relative to the normal of the rotor plane, the angle- of-attacks and lift at the rotors 102 are different for different azimuth angles of the one or more blades similarly to horizontal wind shear. Thus, for azimuth angles of the one or more rotor blades 103 corresponding to locations of a rotor blade 103 near the 3 o'clock and 9 o'clock positions, a difference of the pivot angles a are generated due to different lift values generated by the vertical inflow.

The horizontal wind shear and the vertical inflow of the wind may be determined based on a mean pivot angle parameter difference Aa_m determined based on third and fourth mean parameter values a3_m, a4_m, e.g. as Aa_m = a3_m- a4_m.

The third mean pivot angle parameter value a3_m is determined based on the pivot angle signal 401 obtained when one of the blades points sideways (e.g. near the 9 o'clock position) and is perpendicular with or makes an acute angle greater than 45 degrees to the longitudinal direction 601 of a tower 101. The fourth mean pivot angle parameter value a4_m is determined based on the pivot angle signal obtained when one of the blades points in the opposite sideways direction (e.g. near the 3 o'clock position) and is perpendicular with or makes an acute angle greater than 45 degrees to the longitudinal direction of the tower 101.

Each of the mean pivot angle parameter values al_m-a4_m may be determined as an average of a plurality of values of the pivot angle signal 401 for a specific blade 103, or for two or more of the blades of the rotor 102, when the blade(s) pass a given azimuth angle or are within an azimuth range. Further, each of the plurality of values of the pivot angle signal 401 may be determined from a single sample of the pivot angle signal 401 obtained at a relevant azimuth angle, or may be determined as a mean value of samples of the pivot angle signal 401 over a range of the azimuth angles. Filtering may be used in the determination of the mean pivot angle parameter values al_m-a4_m to remove undesired frequency content. The mean pivot angle parameter values al_m-a4_m may be obtained over different periods to distinguish between vertical and horizontal wind shear and horizontal and vertical inflow. Thus, long term differences in mean pivot angles parameters at 12 and 6 o'clock over e.g. one hour, 6, 12 hours or weeks could indicate vertical wind shear. Vertical wind shear can change between day and night which will cause a change in the mean pivot angle parameters at 12 and 6 o'clock between day and night operation. The level of difference between the mean pivot angle parameters at 12 and 6 o'clock indicate the size of the vertical wind shear.

Long term differences in mean pivot angle parameters at 3 and 9 o'clock over months/years in each wind direction sector is due to nacelle tilt or slope (e.g. a hill in front of the wind turbine. Medium long term differences in the mean pivot angle parameters at 3 and 9 o'clock over e.g. one hour, 6, 12 hours or weeks indicates horizontal wind shear, which as well can vary between e.g. day and night. In sectors with a wind turbine in front of the wind turbine, the differences in mean pivot angle parameters at 3 and 9 o'clock depends on if the front wind turbine is in operation or is stopped. A large difference on top of the mean difference of the pivot angle parameters at 3 and 9 o'clock over longer periods due to tilt, slope and horizontal wind shear could indicate large wake effects. If wake effects occurs the control of the front wind turbine can be changed by controlling the front wind turbine to yaw, pitch or pivot out of the wind to reduce the wake effect. Further, in case of operation with large wake effects the shadow wind turbine may be de-rated or yawed a bit out of the wind to reduce the loads.

The wind shear and inflow slopes may be caused by wake effects due to shadowing effects of other wind turbines, by buildings, hills and other environmental effects. Specifically, the horizontal inflow may be caused by a yaw error of the wind turbine.

The different wind field inflow parameters for a given wind turbine may be used for determining mean values of these wind field inflow parameters, such as mean values over different periods of time, e.g. a year, a month, a day, hours or minutes. The mean values may further be determined for different intervals of orientations of the rotor (i.e. different angular ranges in a wind rose), or equivalently for different yaw angle intervals. In this way it is possible to characterize the wind field inflow parameters of the wind turbine for different wind direction intervals corresponding to the different yaw angle intervals.

The determined mean values of the wind field inflow parameter for one or more of the yaw angle intervals may be compared with historic values of corresponding mean values of the wind field inflow parameter obtained for the same one or more yaw angle intervals. A possible deviation resulting from the comparisons may indicate a fault and may be used for determining a control parameter for the wind turbine or another wind turbine.

For example, it may be determined that a deviation of the mean value of the wind field inflow parameter is due to a wake effect of another wind turbine and, therefore, the operation of the other wind turbine may be modified to reduce the impact of the wake effect. For example, if it can be ascertained that the horizontal wind shear is due to wake, this could be used to change control parameters of the up-wind wind turbine 100.

A determined horizontal inflow which deviates from an expected horizontal inflow may indicate a yaw error. Thus, the determined horizontal inflow can be used to control the yaw system to reduce the horizontal inflow. Alternatively, the pivot force can be reduced resulting in increased pivot angles which again results in that the blades in an increased way will operate as a wind vane, i.e. so that the wind on the blades will generate a yaw moment that for a free yawing wind turbine will rotate the wind turbine into the wind - to reduce the horizontal inflow.

Other examples of control actions generated in response to the wind field inflow parameter comprises: shutdown of one or more wind turbines if a wind field inflow parameter is greater than a threshold, e.g. if the wind turbulence parameter or wind shear is above a given threshold, and reduction of power production and/or rotor speed if the wind field inflow parameter, e.g. turbulence or wind shear, is greater than a threshold. For example, the wind turbulence parameter may be used to control the pivot angle a so that high turbulence levels results in larger pivot angles and less rotor area to reduce loads, whereas low turbulence levels results in small pivot angles, larger rotor area and larger power production.

The above-mentioned threshold may be specified for different yaw angle intervals. For example, the threshold may be derived from historic average values of different yaw angle intervals order to determine deviating wind field inflow parameters.

The control of the power production and/or rotor speed may be performed by controlling the pivot angle based on the determined wind field inflow parameter.

Accordingly, the wind turbine may comprise a control system arranged to control the wind turbine dependent on the determined wind field inflow parameter. The control system comprises functions for controlling the pivot angle a dependent on the wind field inflow parameter and is configured to control the pivot angle dependent on wind field inflow parameter and other parameters such as a power reference. The wind field estimator 400 may be arranged for determining control signals dependent on the determined wind field inflow parameter, or to determine control signals for a separate control system as described above.

The scope of the present invention is to be interpreted in the light of the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. A method for determining a wind field inflow parameter based on monitoring parameters of a wind turbine (100) with a variable rotor area, the wind turbine comprises a rotor (102) with one or more rotor blades (103) which are arranged hinged at a variable pivot angle (a), where the variable rotor area depends on the pivot angle, and where the pivot angle is dependent on a variable pivot force (F), the method comprises,

- determining a pivot angle signal (401) which relates to the pivot angle of at least one of the rotor blades (103), - determining the wind field inflow parameter (402) based on the pivot angle signal.

2. A method according to claim 1, wherein the method comprises determining a standard deviation and/or a mean value of the pivot angle signal.

3. A method according to claim 2, wherein the wind field inflow parameter comprises a wind turbulence parameter determined based on a standard deviation of the pivot angle signal (401) and an obtained air density.

4. A method according to any of claims 2-3, wherein the wind field inflow parameter comprises a mean wind speed determined based on a mean value of the pivot angle signal (401) and the air density.

5. A method according to any of the preceding claims, wherein the method comprises determining a mean pivot angle parameter difference based on mean pivot angle parameter values of the pivot angle signal (401) determined for different azimuth angles of the one or more blades.

6. A method according to claim 5, wherein a first of the mean pivot angle parameter values is determined based on the pivot angle signal (401) obtained when one of the blades points upwards and is parallel with or make an acute angle less than 45 degrees to a longitudinal direction of a tower of the wind turbine, and wherein a second of the mean pivot angle parameter values is determined based on the pivot angle signal (401) obtained when one of the blades points downwards and is parallel with or make an acute angle less than 45 degrees to the longitudinal direction of a tower of the wind turbine.

7. A method according to any of claims 5-6, wherein a third of the mean pivot angle parameter values is determined based on the pivot angle signal obtained when one of the blades points sideways and perpendicular with or makes an acute angle greater than 45 degrees to a longitudinal direction of a tower of the wind turbine, and wherein a fourth of the mean pivot angle parameter values is determined based on the pivot angle signal obtained when one of the blades points in the opposite sideways direction and is perpendicular with or makes an acute angle greater than 45 degrees to the longitudinal direction of a tower of the wind turbine.

8. A method according to any of claims 5-7, wherein the wind field inflow parameter comprises a vertical wind shear and/or horizontal inflow of the wind determined based on the mean pivot angle parameter difference, such as the mean pivot angle parameter difference determined based on the first and second mean pivot angle parameter values.

9. A method according any of claims 5-8, wherein the wind field inflow parameter comprises a horizontal wind shear and/or vertical inflow of the wind determined based on the mean pivot angle parameter difference, such as the mean pivot angle parameter difference determined based on the third and fourth mean pivot angle parameter values.

10. A method according to any of the preceding claims, comprising determining a mean value of the wind field inflow parameter for different yaw angle intervals.

11. A method according to claim 10, comprising comparing the mean value of the wind field inflow parameter for one or more of the yaw angle intervals with historic values of a corresponding mean value of the wind field inflow parameter obtained for the same one or more angle intervals and determining a control parameter for the wind turbine or another wind turbine based on the comparison.

12. A method according to any of the preceding claims, comprising controlling the wind turbine based on the determined wind field inflow parameter.

13. A method according to claim 12, wherein controlling the wind turbine comprises adjusting the pivot force (F) based on the determined wind field inflow parameter.

14. A wind field estimator (400) arranged to perform the steps according to the method of claims 1-13.

15. A wind turbine (100) comprising a rotor (102) with a variable rotor area, where the rotor comprises one or more rotor blades (103) which are arranged hinged at an adjustable pivot angle (a), where the variable rotor area depends on the pivot angle, and where the pivot angle is adjustable dependent on a variable pivot force (F) provided by a pivot actuator (301), and the wind field estimator (400) according to claim 14.