Method for load reduction on wind turbine blade pitch bear ^¬ ings

Field of invention

The present invention relates to a method and arrangement for adjusting a pitch angle of a rotor blade of a wind turbine and further relates to a wind turbine comprising the arrange ^¬ ment .

Art Background A wind turbine comprises a rotor at which plural rotor blades are connected and which drive a generator to generate elec ^¬ tric energy. The rotor blades may be adjusted with respect to their orientation relative to the wind wherein the rotor blades may be rotated around a longitudinal axis of the rotor blades which is commonly referred to as pitching the rotor blade. Modern wind turbines may be designed for pitch regula ^¬ tion using either pneumatic, hydraulic or electric pitch sys ^¬ tems. Thereby, conventionally, a pitch system may consist of an actuator which may transform a signal coming from the con- troller into a motion signal for the pitch motor/pitch actuator which in turn rotates the blade around its pitch axis (i.e. the longitudinal axis of the rotor blade) .

The blade and the pitch system may be coupled through a pitch bearing. During operation, the pitch bearing may be subjected to high bending loads due to the bending of the blades into the wind. The highest blade thrust may occur at the wind speeds just before rated power is reached and just after rat ^¬ ed power is reached. At this time of high loading there is also significant pitch motion to limit those loads and also for power regulation. The bending of the rotor blade may cause elastic deformation internally in the pitch bearing which in turn may cause high wall pressure between the balls of the bearing itself and the runway (a notch along which the bearing balls or cylinders may roll) . When the pitch system is actuated, these high con ^¬ tact pressures inside the bearing may lead to fatigue damage. This eventually may lead to component damage. Particularly hazardous may be the combination of pitch activity and very high bending loads which although are seldom (in terms of relative occurrence) may lead to so-called low cycle fatigue. Blade bearing fatigue may be the product of the bending mo ^¬ ment on the blade taken to an exponential power and the blade pitch movement. Blade bearing fatigue may depend non-linearly on the load, specifically it might be proportional to load elevated to the power of 3 multiplied by the pitch movement. Blade pitch bearing may be a limiting factor in the design of larger wind turbines.

Several methods have been applied, to reduce the contact pressure in the raceway or runway of the pitch bearing: Con ^¬ ventionally, the loading on the turbine can be reduced. This may be done by what is sometimes called "peak shaving". Peak shaving may be a change which is introduced into the control ^¬ ler behaviour such that the blades pitch earlier into the wind, in this way reducing the loading on the whole rotor. The drawback of this methodology is that it may also affect the power performance and thus reduces the energy production.

Another methodology to reduce bearing load may be to increase the size of the bearing. Increasing the diameter of the bearing may reduce the local induced stresses in the material. Nevertheless, this solution may not be very practical, once the hub and the blades have been designed. A larger pitch bearing may also be more expensive, increasing the cost of the generated energy.

Further methods which have conventionally been used may in ^¬ clude for example partial pitch, wherein the pitch system is moved outwards along the blades while retaining a fixed pitch section inboards, or cyclic pitch, which reduces extreme loads, but increase pitch activity. This may be a complicated interaction. Also further outboard, the bearing would need to be smaller and have a lower fatigue limit.

Document US 2011/0305568 Al discloses a wind turbine control system suitable for minimizing actuation of pitch actuators, wherein an error gain schedule in full load control is used for reducing pitch actuation when the difference between the rotor speed and the reference rotor speed is not critical for the load of the wind turbine components. The use of the error gain schedule may reduce wear of the pitch actuators and may improve reduction of structural oscillations, since focus is removed from tracking the rotor speed reference when the speed error is low.

It has however been observed, that conventional methods and arrangements are not in all situations and under all circum- stances capable of reliably and effectively reducing pitch bearing load or pitch bearing damage, while at the same time other components are not negatively affected or the wind tur ^¬ bine performance is not negatively affected. Thus, there may be a need for a method and arrangement for adjusting a pitch angle of a rotor blade, wherein a load on the pitch bearing is reduced, while at the same time other components are not negatively affected and the performance of the wind turbine is not negatively affected.

Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to an embodiment of the present invention

provided a method for adjusting a pitch angle of a blade connected to a rotor of a wind turbine, the method com ^¬ prising pitching the rotor blade towards a target blade pitch angle, the (in particular way of, or manner of, or characteristic, or speed of) pitching depending on a (e.g. measured, or simulated, or estimated, or theoretically calculated) load on a pitch bearing, in particular depending on an azimuthal position of the rotor.

According to an embodiment of the present invention it is provided a method (or arrangement) for adjusting a pitch an ^¬ gle of a rotor blade connected to a rotor of a wind turbine, the method comprising pitching the rotor blade towards a target blade pitch angle, the (in particular way of or manner of or characteristic or speed of) pitching depending on an azi- muthal position of the rotor. Thereby is may not be necessary to determine a load on the bearing.

The load on the pitch bearing may vary or correlate with the azimuthal position of the rotor. The load on the pitch bear- ing may further vary or correlate with factors (like motion direction of rotor blade, external conditions etc.) other than the azimuthal position of the rotor. These additional input sources could include but not limited to wind speed measurements, tower strain and torsion, power measurement, pitch actuator strain, blade vibration, tip deflection sensor, Lidar ("laser illuminated detection and ranging") measurements, shear measurements, or any other measurement that can be correlated to pitch bearing load. The method may be performed by a controller and/or a pitch adjustment system of a wind turbine. The method may be ap ^¬ plied independently and separately for each of plural rotor blades connected to the rotor. Thus, the different rotor blades may be pitched independent from each other and in par- ticular in a different manner. Thereby, a target pitch angle of all rotor blades may be the same but the manner or the way in which the different rotor blades are pitched towards the target pitch angle may be different. The manner whether to pitch or how to pitch (with what pitch speed) the rotor blades may depend on the azimuthal position of the rotor, thus also on the azimuthal position of the respective rotor blade or the orientation of the rotor blade.

For each of the plural rotor blades, the azimuthal position of the rotor may be defined for example relative to a verti ^¬ cal position of the respective rotor blade in which the top of the rotor blade points upwards. Depending on the azimuthal position of the rotor, pitching may be enabled, disabled or performed with a particular pitch rotational speed which may vary for different azimuthal positions. In particular, when the rotor blade is oriented in an azimuthal range which is subjected to relatively high loads, the pitching may either be disabled in this region or the pitching speed may be re ^¬ duced relative to other azimuthal positions. Further factors may govern the manner how the pitching is performed. For example, environmental factors, such as wind speed, wind turbu ^¬ lence and/or an actual measured or estimated load on the ro- tor blade and/or on the pitch bearing may be taken into account to affect the manner in which the pitching is performed .

The target blade pitch angle may be the blade pitch angle op- timal for the actual rotational speed of the rotor and opti ^¬ mal for the present wind speed and wind direction, in order to extract maximum electric energy from the wind. During a full revolution of the rotor blade, the manner in which the rotor blade is pitched may depend on the azimuthal position. The pitch bearing damage may be calculated or estimated e.g. as the product between the pitch travel and the bearing mo ^¬ ment elevated to a given power exponent. In particular, when the bearing moment is relatively high, the pitch travel may be reduced, in order to reduce the pitch bearing damage. The pitch travel may for example be reduced by disabling to pitch the rotor blade in a range where the bearing moment is rela ^¬ tively high or in particular higher than a threshold or by reducing the pitch rotational speed in this region. In particular, pitching the rotor blade may be performed such that the pitch bearing damage is substantially equal for a whole revolution of the rotor blade. Other schemes are possi- ble. Thereby, the pitch bearing damage may be effectively re ^¬ duced, while ensuring high wind turbine performance and with ^¬ out damaging other components of the wind turbine.

According to an embodiment of the present invention, pitching of the rotor blade comprises disabling pitching the rotor blade in a range of azimuthal positions of the rotor, in par ^¬ ticular where a load on the rotor blade pitch bearing is above threshold, enabling pitching the rotor blade in another range of azimuthal positions of the rotor, in particular where a load on the rotor blade pitch bearing is at or below the threshold.

When the pitching is disabled in a range of azimuthal posi ^¬ tions, the pitch travel is zero in this region, thereby also pitch bearing damage may be very low or even zero in this range. When pitching the rotor blade is enabled in another range of azimuthal position where a load is relatively low and in particular below the threshold, the pitch bearing damage can be ensured to be in an acceptable range. Pitching the rotor blade may in particular be disabled in a range of azi ^¬ muthal position in which the rotor blade points upwards (or is performing downswing) or is in a range between +/- 60° or +/- 40° deviating from an orientation between 30° and 50° in the direction of downswing. Other ranges are possible. In this range, the bending moment of the rotor blade or the bearing bending moment may be expected to be highest, since in this range, the rotor blade is arranged at highest alti ^¬ tude where the highest wind speed is expected and/or where the highest gravitation or lever arm acts on the blade and/or pitch bearing.

The most damage to the pitch bearing may occur on the down ^¬ swing. This is when gravity, torque forces and thrust forces may cause the most damage to the weakest spot on the bearing. At average shear it may be about 80° to 100° azimuth angle. Pitching may be limited to be performed past downswing to be ^¬ fore vertical position.

According to an embodiment of the present invention, pitching the rotor blade comprises pitching the rotor blade with a pitch rotational speed that depends on the azimuthal position of the rotor, wherein in particular the pitch rotational speed is adjusted to be lower for an azimuthal position in which the longitudinal axis of the rotor blade is vertically oriented, while a tip of the rotor blade is pointing upwards, than for other azimuthal positions.

The pitch rotational speed may be the rotational speed with which the rotor blade is rotated around its longitudinal ax ^¬ is. When the pitch rotational speed is low, the pitch travel (in a particular time interval) is low, when the pitch rotational speed is high, the pitch travel (in this particular time interval) is high. Thus, for example by reducing the pitch rotational speed in a particular range of azimuthal po ^¬ sitions, results in decreasing the pitch travel in this re ^¬ gion. In particular, the pitch rotational speed may be lower in an azimuthal angle range in which the rotor blade is be ^¬ tween +/- 60° or +/- 40° deviating from an orientation between 30° and 50° in the direction of downswing, than in other azimuthal positions or in another azimuthal angle range. In particular, the pitch rotational speed may be inversely proportional an exponent of the (measured or estimated) bear ^¬ ing moment according to particular embodiments.

According to an embodiment of the present invention, the pitch rotational speed depends on the azimuthal position ac ^¬ cording to a predefined curve and/or the pitch rotational speed is inversely proportional to a load on the blade pitch bearing or rotor blade. The predefined curve may have been established by performing simulations or calculations for example in order to keep the pitch bearing damage or pitch bearing load across a whole revolution of the rotor blade below a threshold. The prede- fined curve may be different from a constant. The predefined curve may comprise at least one minimum and at least one max ^¬ imum that are different from each other.

The disabling the pitching in a particular range, the ena- bling the pitching in another range of azimuthal position and the pitching the rotor blade with a pitch rotational speed that depends on the azimuthal position on the rotor may be combined .

According to an embodiment of the present invention, the pitch rotational speed is different for the cases where the target pitch angle is adjusted from an initial pitch angle to the target pitch angle in a direction towards and away from stall. Simulation or physical modelling may enable to define whether the rotational speed is higher or lower when the initial pitch angle is adjusted in a direction towards or away from stall. Defining the pitch rotational speed may depend on other factors, such as wind speed, wind turbulence, rotation ^¬ al speed of the rotor and the like.

According to an embodiment of the present invention, the pitching the rotor blade comprises pitching the rotor blade depending on a measured blade root moment and/or pitch bearing load or expected pitch bearing damage indicating quantity.

The pitch bearing load may be estimated from operational pa ^¬ rameters of the wind turbine or the environment, such as wind speed, geometry and material of the rotor blade and/or actual pitch angle and so on or may be measured using one or more sensors, such as strain sensors. The measured blade root mo ^¬ ment and/or the pitch bearing load may turn out to also de- pend on the azimuthal position of the rotor. Thereby, an ef ^¬ fective reduction of the bearing load may be achieved.

According to an embodiment of the present invention, the pitching the rotor blade comprises pitching the rotor blade further depending on pitch bearing load accumulated over time. When the pitch bearing load accumulated over time is relatively low, the pitch bearing may sustain and withstand relatively high future loads. When the pitch bearing load ac- cumulated over time is relatively high, future bearing loads may be reduced by for example disabling the pitching in a particular azimuthal position range or at least reducing the pitch rotational speed in this range or these ranges. According to an embodiment of the present invention, the azi ^¬ muthal position is defined in degrees relative to the verti ^¬ cally oriented longitudinal axis of the rotor blade, while the tip of the rotor blade is pointing upwards, wherein the pitch rotational speed has a first average value in an azi- muthal position range between 80° and 350°, in particular be ^¬ tween 110° and 330°, wherein the pitch rotational speed has a second average value in an azimuthal position range between 300° and 140°, in particular between 330° and 110°, wherein the first average value is between two times and 100 times as great as the second average value. No pitching may be per ^¬ formed between 0° and 90°.

The azimuthal position may be individually defined for every rotor blade of plural rotor blades connected to the rotor. The curve according to which the pitch rotational speed may be set may be symmetric or not symmetric with respect to the position at 0° or at 180°. Simulations and physical modelling or excremental data may be performed to define the curve de ^¬ fining the pitch rotational speed in dependence of the azi- muthal position of the rotor blade, such as e.g. arrive at a desired course of bearing load across a whole revolution of the rotor blade. According to an embodiment of the present invention, the pitch rotational speed depends on the azimuthal position, if pitch bearing load indicating quantity indicates that the load of the pitch bearing, on average or when vertically ori- ented, exceeds a threshold. If it turns out that the pitch bearing load is, even when the rotor blade is vertically ori ^¬ ented and pointing upwards, relatively low, such as below a threshold, it may not be necessary to disable pitching the rotor blade or it may not be necessary to reduce the pitch rotational speed in a particular azimuthal position range. Thereby, the rotor blade may be faster pitched towards the target blade pitch angle.

According to an embodiment of the present invention, the pitch bearing load indicating quantity is estimated and/or measured and/or calculated from at least one operational pa ^¬ rameter of the wind turbine or the environment. The pitch bearing load may for example be estimated or calculated from wind speed, rotor blade geometry and material, actual blade pitch angle and/or characteristics of the pitch bearing, such as geometry, material, design and so on. Thereby, the pitch ^¬ ing of the rotor blade may also be performed directly depend ^¬ ing on the pitch bearing load (as estimated or calculated or measured) .

The pitch rotational speed may depend on the average load of each blade's pitch bearing, wherein in particular the rate of pitch change is a function of each blade's bearing load compared to the average bearing load on all blades. The pitch bearing loads M _PitB earFi _U ctA, M _PitB earFi _U ctB, M _P i _tB earFi _U ctc of blades A, B and C, respectively (depending on which the pitching may be performed) may e.g. be determined based on the following equations (1) to (4), wherein M _Pit BearMean is the average load over all blades. WpitBearMean ~ (^PitBearA + ^PitBearB + M _PitBearC )/3 [kNlTl] (1)

^PitBearFIuctA ⁼ ^PitBearA ~~ ^PitBearMean [kNm] (2)

WpitBearFiuctB ⁼ ^PitBearB ^— WpitBearMeaii [kNlTl] (3)

^• WpitBearFiuctC ⁼ ^PitBearC ^— ^PitBearMean [kNm] (4)

Thus the individual pitch bearing loads (as used in embodi ^¬ ments of the present invention) of the blades may be calcu- lated as a difference between the average pitch bearing load and the measured or estimated pitch bearing load M _P i BearA, pitBearB, _Pit BearB of the individual blade.

According to an embodiment of the present invention, pitch rotational speed depends on the load on the pitch bearing and/or the azimuthal position, if: the load level of pitch bearing exceed a threshold, and/or a wind turbulence level is greater than a threshold, and/or a deviation of a drive train rotational speed from a reference exceeds threshold and/or a wind speed is greater than a threshold.

In those situations or under those conditions, the pitch bearing load may be at risk to be above a threshold in one or more azimuthal angle ranges. In these azimuthal angle ranges, the pitch rotational speed may then be reduced to satisfy one criterion, such as that the pitch bearing damage or pitch bearing load is below a threshold.

The pitching the rotor blade may comprise rotating the rotor blade around a longitudinal axis of the rotor blade.

According to an embodiment of the present invention, it is provided a method for adjusting pitch angles of a first rotor blade and a second rotor blade both connected to a rotor of a wind turbine, the method comprising pitching the first rotor blade towards a target blade pitch angle in a different man ^¬ ner than pitching the second rotor blade towards the target blade pitch angle. Pitching each of the first rotor blade and the second rotor blade may be performed independently from each other accord ^¬ ing to one of the embodiments of a method for adjusting a pitch angle of a rotor blade as described above.

It should be understood that features, individually or in any combination disclosed, described, or provided for a method for adjusting a pitch angle of a rotor blade may also be ap- plied, individually or in any combination, to a method for adjusting pitch angles of a first rotor blade and a second rotor blade both connected to a rotor of a wind turbine and may also applied to an arrangement for adjusting a pitch an ^¬ gle of a rotor blade connected to a rotor of a wind turbine according to an embodiment of the present invention and vice versa .

According to an embodiment of the present invention, it is provided an arrangement for adjusting a pitch angle of a ro- tor blade connected to a rotor of a wind turbine, the ar ^¬ rangement comprising: a pitching system adapted to pitch the rotor blade towards a target blade pitch angle, the manner to pitch depending on an azimuthal position of the rotor. The pitch system may comprise an actuator enabled to rotate the rotor blade around its longitudinal axis. The pitch sys ^¬ tem may receive (the target pitch angle and) the azimuthal position of the rotor or the azimuthal position of the rotor blade connected to the rotor and may pitch the rotor blade depending on the azimuthal position of the rotor or the azimuthal position of the rotor blade or the orientation of the rotor blade towards the target pitch angle.

Further, a wind turbine is provided which comprises a rotor, at least one rotor blade connected to the rotor and an ar ^¬ rangement for adjusting a pitch angle of a rotor blade ac ^¬ cording to an embodiment as described above. It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with ref ^¬ erence to method type claims whereas other embodiments have been described with reference to apparatus type claims. How ^¬ ever, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features re- lating to different subject matters, in particular between features of the method type claims and features of the appa ^¬ ratus type claims is considered as to be disclosed with this document . The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi- ment but to which the invention is not limited.

Brief Description of the Drawings

Embodiments of the present invention are now described with reference to the accompanying drawings. The invention is not restricted to the illustrated or described embodiments.

Fig. 1 schematically illustrates a wind turbine according to an embodiment of the present invention;

Fig. 2 illustrates a graph showing a wind speed experienced by several rotor blades.

Fig. 3 illustrates a graph of the blade root bending moment in dependence of the azimuth angle; Fig. 4 illustrates a plot of the pitch bearing accumulated damage in dependence of the azimuth angle and the bending mo ^¬ ment ; Fig. 5 illustrates a graph showing the accumulated damage at high loading in dependence of the rotor azimuthal position;

Fig. 6 illustrates an example of a pitch speed scaling factor in dependence of the azimuthal position;

Fig. 7 illustrates a graph showing the accumulated pitch dam ^¬ age in dependence of the bending moment for different wind turbulences; and Fig. 8 illustrates graphs of the bearing moment and the pitch position in dependence of time.

Detailed Description The illustration in the drawings is in schematic form.

The wind turbine 1 illustrated in Fig. 1 comprises a rotor 3 at which plural rotor blades 5 are connected. The rotor 3 drives a generator 14 which generates, upon rotation of the rotor 3, electric power (for example 3-phase power) which may be supplied to an (optional) AC-DC converter from there to a (optional) DC link and from there to (optional) a DC-AC con ^¬ verter which converts the energy stream to a fixed frequency power stream which is supplied to a transformer 13 which transforms the power stream to a higher voltage and which is connected to a point of common coupling 15 to which further wind turbines are connected. The point of common coupling is optionally via a wind park transformer 17 connected to a utility grid 19.

The wind turbine 1 further comprises an arrangement 21 for adjusting a pitch angle a of the rotor blade 5 according to an embodiment of the present invention. The arrangement 21 comprises a pitching system adapted to pitch the rotor blade towards a target blade pitch angle dependent on an azimuthal position β of the rotor 3. Therein, the azimuthal position β of the rotor 3 is defined with respect to a rotation around the rotor rotation axis 4 as indicated by the arrow 6. Thus, the pitching system 21 pitches the rotor blade, i.e. rotates the rotor blade 5 around its longitudinal axis 8 along the pitching direction as indicated by the arrow 10 which pitching depends on the orientation or azimuthal position β of the rotation of the rotor 3 around the axis 4.

The inventors noted that the combination of pitch activity and very high bending loads may lead to pitch bearing damage and in particular may lead to a so-called low cycle fatigue. Embodiments of the present invention reduce a load on the pitch bearing 12 which supports the rotor blade 5 rotational- ly enabling pitching the rotor blade 5 around the longitudi ^¬ nal axis 8 of the rotor blade 5. Embodiments of the present invention utilize the fact that the damage of the pitch bear ^¬ ing 12 is not accumulated uniformly along the azimuth (i.e. along one revolution of the rotor 3) , but that more damage accumulates when the blades 5 are pointing upwards and/or in a range of azimuthal position where the blade is at down ^¬ swing. The rotor blades 5 have a root 16 at which the rotor blades 5 are connected or mounted at the rotor 3, in particu ^¬ lar mounted at a hub of the rotor 3. The bearing may be lo ^¬ cated close to the root 14.

A simple explanation for this observation is the distribution of blade loads as a function of the azimuth. Normally, the wind speed in the top of the rotor is expected and is in fact higher than in the lower part. Fig. 2 illustrates an example of wind speed measurements, wherein the axis 23 denotes a horizontal position and the axis 25 denotes a vertical posi- tion, while the measured wind speed is depicted as a heat map. Shown are three rotor blades 5a, 5b, 5c which are at different orientations and in which two (5a, 5c) of them ex ^¬ perience higher wind speed and thus higher loads than the third rotor blade 5b. The heat map shows the trend of higher wind speeds towards the top. This in turn causes a higher loading at the blade-hub interface as shown in the next Fig. 3, in which the abscissa 27 indicates the azimuth angle (i.e. an angle β along the arrow 6 defining an azimuthal position of the rotor 3) and wherein the ordinate 29 denotes the blade root bending moment. The curve 31 of the blade root bending moment indicates that the loading is highest when the azimuth is around 0° (or 360°) which corresponds to the position of the blade pointing upwards.

When combining the full range of operation covering wind speeds from cut-in to cut-out, considering the different lev ^¬ els of loading and computing the accumulated damage, a sur- face plot as illustrated in Fig. 4 is obtained. Therein, the axis 33 denotes the bending moment, the axis 35 denotes the azimuthal position and the vertical axis 37 denotes the pitch bearing accumulated damage. The surface plot 39 shows the damage distributed into bending moment bins and azimuth bins. Damage is accumulated in the form of duration loads, where damage occurs only when the pitch system is active.

If a cut is made at high loading levels along the surface of Fig. 4, then an accumulated damage distribution as the one shown in Fig. 5 is obtained, wherein the abscissa 41 indi ^¬ cates the rotor azimuthal position, while the ordinate 43 de ^¬ notes the accumulated damage at high loading. The curve 45 illustrating the accumulated damage at high loading shows that the most of the damage is accumulated when the blades are in between the upper part (vertically high) of the rota ^¬ tion plane and continuing until past the downswing. The damage is shown most from 0° to 130°.

According to one embodiment of the present invention, the ac- cumulated pitch damage is reduced by reducing the pitch ac ^¬ tivity when the blades are in the upper section of the rotor plane. Reducing the pitch activity may be performed in a num ^¬ ber of ways, as is explained below. In one embodiment of the present invention, the pitch speed of every blade is scaled independently according to the local azimuth position according to a prescribed function similar to that what is shown in Fig. 6, wherein the abscissa 41 de ^¬ notes the azimuthal position, while the ordinate 44 denotes the pitch speed multiplicative factor, i.e. a scaling factor according to which the pitch rotational speed is adapted. The form of the prescribed function 47 showing the pitch speed multiplicative factor may be different for pitch movement to ^¬ wards and away from stall, thus when the upper edge of the blade is turned away from the tower so that the blade is in the wind or such that the upper edge of the rotor blade faces the wind.

According to another embodiment, the loads on the pitch bearing may either be estimated from convenient signals or meas ^¬ ured by a sensor, such as a strain gauge sensor or optical fibre sensor, positioned at the root 14 of the blade 5 (see Fig. 1) . The sensor may also be mounted at an outboard sec ^¬ tion of the blade. When the level of loading exceeds a given threshold, the scaling of the pitch speed may begin. Thus, the definition of the pitch speed may be according to the curve 47 illustrated in Fig. 6 depending on the actual bend- ing moment or bearing load.

Fig. 7 illustrates curves 32, 34, 36 of accumulated pitch damage at wind turbulence intensities 0.08, 0.1, 0.12 respec ^¬ tively in dependence of the bending moment denoted on the ab- scissa 30, while the ordinate 28 denotes the accumulated pitch damage. As can be observed from the curves 32, 34, 36 in Fig. 7, the accumulated pitch damage increases with in ^¬ creasing wind turbulence intensities. Embodiments of the pre ^¬ sent invention aid to reduce pitch bearing damage.

In still another embodiment of the present invention, a char ^¬ acteristics of the temporal and/or spatial distribution of the incoming turbulent flow may be estimated using convenient signals and the pitch velocity may be adapted accordingly (i.e. less pitch actuation in the upper part of the rotor at high turbulence intensity levels than at low level, more pitch actuation or higher pitch speed in whichever area of the rotor the axial wind speed is lower or lowest) .

In still another embodiment of the present invention, the pitch speed scaling factor may be adjusted in response to a signal derived from the difference between drive-train rota- tional speed and a reference, such that all blade pitch, ir ^¬ respective of azimuth, in cases where the drive-train speed deviates greatly from the reference speed.

In still another embodiment of the present invention, the pitch speed scaling factor may be adjusted in response to a wind speed measurement or estimate, set of wind speed meas ^¬ urements or wind shear measurement or estimate upstream from the wind turbine. Different features of different embodiments may be combined with those of still other embodiments of the present inven ^¬ tion described below.

The pitch velocity may be dependent on the azimuthal position of the rotor or rotor blade and may potentially also depend on the turbulence intensity level of the wind.

According to embodiments of the present invention, the fact is used that the damage of the pitch bearing is not accumu- lated uniformly at all azimuth angles (i.e. along one rota ^¬ tion of the rotor) .

Embodiments of the present invention also propose to pitch different blades independently from each other to reduce pitch bearing damage. When a particular blade is under significant more load than the other blades, such as two blades, the higher loaded blade may not be pitched. Since loads on the blades are generally cyclical around the azimuth, this usually means that the blade may not be pitched during around 33% of each rotation or revolution (or for about 1 to 2 s each revolution) . The blade pitch angle of the not pitched (whose pitch angle had been held to reduce pitch bearing load) blade may then catch up to the other blades once the load is lower.

Embodiments of the present invention may be implemented as controllers which serve to reduce pitch bearing load.

According to one embodiment of the present invention, the pitch rate (i.e. the pitch speed) is reduced for the more heavily loaded blades relative to the other blades, as is il ^¬ lustrated in Fig. 8. In Fig. 8, the abscissas 49 of the graphs 48 and 50 denote the time, while the ordinate 51 de ^¬ notes the bearing moment and the ordinate 53 denotes the pitch position. In graph 48 of Fig. 8 the curve 52 indicates the bearing moment of a first blade, the curve 54 denotes the bearing moment of a second blade, and the curve 56 denotes the bearing moment of a third blade. As it can be observed, the bearing moments vary with time, as the blades rotate around the rotor axis. Thus, the bearing moments depend on the azimuthal position. The pitch rate or pitch speed of pitching those rotor blades which are at azimuthal positions showing high bearing moment may be reduced relative to rotor blades being subjected to lower bearing moments, i.e. being at other azimuthal positions.

According to another embodiment of the present invention, in- dividual pitch rates or pitch speeds of the blades may be set inversely proportional to the load on each blade, wherein the load is for example been taken from measured bearing moments as illustrated in graph 48 of Fig. 8. When the bearing moment is relatively high, the pitch rate or pitch speed on the respective rotor blade may be limited or may be reduced relative to other rotor blades. Graph 50 of Fig. 8 illustrates curves 58, 60, 62 of the pitch position of the first, the second and the third rotor blades, respective ^¬ ly, in dependence of time. It is most clear from the time in ^¬ terval between 14 and 20 s of the simulation, that when the bearing moment is high, the pitch rate on that blade is lim- ited. The slope of the line in graph 50 is the pitch rate. When there is no slope (slope is zero) , the blades are not being pitched. They are not being pitched when the loading in graph 48 is high. According to still another embodiment of the present inven ^¬ tion, the pitch motion may be disabled as a function of the azimuth angle. For example, a rotor blade which is under high shear, may be disabled from pitching when for example point ^¬ ing upwards .

According to still another embodiment of the present inven ^¬ tion, a wind sensing may be used to change the pitch rates or pitch speeds as a function of the azimuth angle. Furthermore, according to still another embodiment of the present invention, pitch damage is accumulated over lifetime and adjusted accordingly using one of the above-mentioned methods . According to an embodiment of the present invention, wind turbine blades are controlled independently to reduce pitch bearing damage. Embodiments of the present invention target one blade at a time and may be able to reduce the pitch bear ^¬ ing damage with less power loss or load gain. An advantage of embodiments of the present invention is that the bearing dam ^¬ age can be significantly lowered without a reduction in ener ^¬ gy production.

Embodiments of the present invention may utilize blade root moment sensors which may provide a control signal to individ ^¬ ually pitch the blades to reduce pitch bearing damage. Advantages of the present invention may provide increased en ^¬ ergy production from turbines that are conventionally heavily peak shaved, in order to reduce pitch bearing damage. It may also reduce warranty costs due to pitch bearing failures. Al- so pitch bearing costs may be reduced when embodiments of the present invention are applied.

It should be noted that the term "comprising" does not ex ^¬ clude other elements or steps and "a" or "an" does not ex- elude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con ^¬ strued as limiting the scope of the claims.