CONTROLLING A WIND TURBINE USING BLADE PITCH ANGLE SETTING AND BLADE ADD-ON SETTING

Field of invention

The present invention relates to a method and an arrangement of controlling a wind turbine by adjusting a blade pitch an gle and at least one blade add-on of at least one wind tur bine rotor blade having the blade add-on. Furthermore, the present invention relates to a wind turbine including the ar rangement .

Art Background

Conventionally, rotational speed of a wind turbine has been controlled by adjusting a blade pitch angle of the rotor blade. In particular, above a rated wind speed the pitch an gle may have been adjusted to a higher value in order to keep the rotational speed of the rotor at a nominal rotational speed.

However, pitching the blade involves some disadvantages in cluding high load of the blade pitch bearings and load on the hydraulic or electrical pitching system. Loading of these sub-systems or components may be a critical design driver and may add costs to the wind turbine. Thus, it is desired to in troduce concepts with less wear of these sub-systems or com ponents .

Thus, there may be a need for a method and a corresponding arrangement for controlling a wind turbine by adjusting a blade pitch angle and at least one blade add-on of at least one wind turbine rotor blade having the blade add-on. Summary of the Invention

According to an embodiment of the present invention it is provided a method of controlling a wind turbine by adjusting a blade pitch angle and at least one blade add-on of at least one wind turbine rotor blade having the blade add-on, the method comprising: adjusting a setting of the add-on to meet a control objective while temporarily maintaining a setting of the blade pitch angle.

The method may for example be performed by a wind turbine control or a module of a wind turbine controller. The method may in particular be applied for wind speeds being higher than a nominal wind speed and/or when output power (or torque) reaches or is at a nominal value or setpoint. The method may for example be performed during idling, e.g. when the wind turbine does not supply power to the utility grid and/or when the rotor has very low rotational speed.

The blade add-on may be configured in different manners. The blade add-on may be considered to comprise an active add-on meaning an add-on which may be altered or adjusted regarding its aerodynamically property. The add-on may be installed at a surface or at least exposed at the surface of the rotor blade. The add-on may for example be configured as a spoiler (one-segmented or multi-segmented) or as a flap for example mounted at a rotor blade tip and/or at a trailing edge of the rotor blade (e.g. a trailing edge flap), e.g. covering radial stations, the flap e.g. being present from beyond midspan to close to the blade tip.

According to embodiments of the present invention, adjusting the adjustable or active blade add-on may be employed for controlling the wind turbine in particular for achieving a desired rotational speed even above a rated or nominal wind speed. Furthermore, primarily, according to embodiments of the present invention, the blade add-on may be adjusted with out at the same time also adjusting the blade pitch angle. Thereby, a load on blade pitch adjusting components such as gears, pitching actuator and so on may be reduced or even avoided .

Particular actuators, involving hydraulic and/or pneumatic and/or electrical actuators may be employed for adjusting the blade add-on. The add-on may for example comprise plural spoiler segments arranged side by side along a longitudinal direction of the rotor blade at a surface of the rotor blade. Each of the spoiler segments may be actuated independently from the other segments. Each segment may for example be ad justed to be on and/or off corresponding for example to an active surface being flipped-out or flipped-in. A flap may also be turned in or turned out to different degrees.

Adjusting the blade add-on may have an effect on the driving impact of the impacting wind, meaning driving impact on the rotational speed or the torque of the rotor blade exerting on the rotor shaft of the wind turbine. Thus, by adjusting the blade add-on, the rotational speed for example of the wind turbine rotor may be controlled, in particular or at least in a particular range for a given (fixed) blade pitch angle.

Adjusting the setting of the add-on will have an influence on the airflow across the rotor blade surface thereby also in fluencing the mechanical torque generated due to the rotor blade surface being altered or due to the airflow being al tered as flowing across the outer surface of the rotor blade. Thereby, also the torque acting on the wind turbine rotor is altered resulting in an effect on the rotational speed of the rotor.

When the control objective can be met by only adjusting the blade add-on without also adjusting the blade pitch angle, wear and load of components of the blade pitch system may be reduced, therefore prolonging the lifetime of those compo nents. Temporarily maintaining a setting of the blade pitch angle may include that (primarily) adjustment of the blade add-on is applied to control or achieve the objective. If the objec tive cannot be met by exclusively adjusting the add-on, then the blade pitch angle may support it with less demands, e.g. only have to adjust the pitch when the blade-add on saturates at limits and/or to more slowly/rarely adjust the blade pitch to get the blade add-on to the right operating point (e.g. ensure that it is not kept in saturation, but moved to a po sition where it is able to control for both increasing and decreasing speed/wind)

During controlling the blade pitch angle setting may tempo rarily be maintained but otherwise utilized to ensure excess capacity in the blade-add on setting by adapting to the cur rent operating point as well as assisting the blade-add on setting in achieving the control objective in certain scenar ios.

The control objective may for example involve to achieve a particular rotational speed of the rotor, and/or to achieve a particular torque of the rotor blade and/or to achieve a par ticular power output. In other embodiments, the control ob jective may involve setting a particular curtailment (i.e. reduction of power below an available power). In these and other embodiments the method may for example be applied for wind speed below (or even above) the nominal wind speed. Whenever it is possible to achieve a control objective by primarily or only adjusting the blade add-on without also ad justing the blade pitch angle, the method may be applied.

Exploiting the controllability of the active blade add-on for different control objectives may result in less pitching and/or better control. This may in particular hold for the case where the control objective is rotational speed of the rotor. According to an embodiment of the present invention an add-on controller as well as a pitch angle controller is employed for implementing the method. The controllers may for example be implemented using conventionally PI controllers.

Wind turbine blades may have active aerodynamic devices (herein also referred to as add-on) installed which may in fluence the aerodynamic properties of the rotor blade. Con ventionally, however blade add-ons may have not been employed for controlling the wind turbine, and particularly have not been employed for controlling rotational speed.

According to an embodiment of the present invention the con trol objective includes achieving and/or tracking a reference rotational speed of the rotor at which the blade is mounted, wherein the adjustment of the setting of the add-on is based on a rotational speed error being a difference between the reference rotational speed and the actual rotational speed.

Achieving and/or tracking the reference rotational speed may include to keep the rotational speed (which may fluctuate due to stochastic wind) close to the reference rotational speed.

The reference rotational speed may correspond to a preset quantity which is for example derived from the mechani cal/electrical/aerodynamical configuration of the wind tur bine. The reference rotational speed may for example corre spond to a design rotational speed with which the wind tur bine is to be operated during normal production operation.

The actual rotational speed may be measured and/or estimated, for example from electrical quantities. Thereby, convention ally available quantities may be utilized for implementing the method.

According to an embodiment of the present invention, the method further comprises adjusting the blade pitch angle based on the rotational speed error and an add-on setting displacement, being a difference between an (reference or ac- tual) add-on setting and a nominal add-on setting, wherein the blade pitch angle is adjusted only if the add-on setting displacement is larger than a displacement threshold and/or if the add-on setting is closer to at least one add-on set ting limit than at least one limit threshold. E.g. if the difference (e.g. remaining capacity) between the add-on set ting and the add-on setting limit is smaller than the limit threshold (threshold on the remaining capacity), the blade pitch angle may be adjusted, since in this case the add-on may not have sufficient capacity for further regulation as it is close to its end adjustment position.

Adjusting the blade pitch angle may involve turning the rotor blade around a longitudinal axis. The add-on may be associat ed with a setting at which the add-on under normal conditions should be operated. The nominal add-on setting may for exam ple correspond to a setting where the add-on has an effect of 50% of its total capacity regarding influencing the aerody namic properties or regarding the airflow around the wind turbine blade. In other embodiments, the nominal add-on set ting may for example correspond to a total off-state of the add-on corresponding to a zero or only a very small effect on the airflow compared to the airflow of the rotor blade having no add-on installed. According to other embodiments, the nom inal add-on setting may correspond to a desired add-on set ting which may be present during normal operation of the wind turbine .

The nominal add-on setting may also be referred to as add-on set point. The add-on set point may be calculated from a re quested add-on utilization factor based on the available add on capacity. Thereby, the utilization factor may be or may comprise :

• a constant,

• a function of turbulence intensity. It could have a con servative (low set point) in high turbulence, leaving higher capacity for the add-ons to deal with wind speed increases which may result in rotational speed increase which may result in over-speed avoidance. In such a case drops in rotational speed may result in possible drops of power being less significant.

• a function of turbine operation values such as power production, idling state, blade load sensor calibration state, rotor balance estimate, or any other turbine state that affects the choice of trade of between over speed avoidance and power production.

Thus, the nominal add-on setting (also referred to as add-on set point) may depend on the particular operational state of the wind turbine as well on environmental factors, such as wind turbulence and/or wind speed. According to a particular implementation of the method, the nominal add-on setting may be set to be a constant in particular or at least in a prede termined turbulence range and/or in a predetermined range of wind turbine operational parameters.

If the add-on setting displacement is relatively large, the add-on setting is far away from the nominal add-on setting.

In this situation, further adjustment of the add-on (at least in a particular direction towards an add-on setting limit) may be not possible anymore or may be possible only in a par ticular margin. In this situation it is advantageous to also adjust the blade pitch angle in particular in such a manner that the add-on setting may be re-adjusted to more closely approach towards the nominal add-on setting again. Thus, ad justing the blade pitch angle does not exclude that also (later on) the add-on is re-adjusted, in particular towards to the nominal add-on setting. The displacement threshold may be set for example as a constant or may also depend on envi ronmental conditions, such as wind turbulence and/or wind speed and/or wind turbine operational data. The limit thresh old may also be set depending on the application.

The add-on setting limit may define a setting of the add-on beyond which the add-on cannot be further moved. So the add on setting limit may define an adjustment border of the pos- sible adjustment of the add-on, such that it is not possible to adjust the add-on beyond the border. For example there may be defined a minimum and a maximum add-on setting limit. If the add-on setting is closer to at least one add-on setting limit than at least one limit threshold (i.e. the difference between the add-on setting and the add-on setting limit is smaller than the limit threshold), it may indicate that the add-on cannot be adjusted anymore sufficiently to achieve the control objective. In this case, it is advantageous to con trol instead or additionally the blade pitch angle. In gen eral, adjusting the blade pitch angle may have a larger ef fect on the torque or the rotational speed than adjusting the add-on.

According to an embodiment of the present invention the nomi nal add-on setting corresponds to a setting from which the add-on can be adjusted in two different/opposing directions (e.g. across similar setting ranges) having differ ent/opposing aerodynamic effect on a blade wind interaction and/or wind driving force and/or airflow around the blade.

When the add-on is at its nominal add-on setting, the add-on may be adjusted to either increase the torque or decrease the torque generated by the air flowing around the rotor blade surfaces and the add-on for effectively controlling the rota tional speed. Thus, trying to keep the add-on at its nominal add-on setting, advantageously may ensure the possibility to control the rotational speed upwards as well as downwards, as needed. Thereby the control of the rotational speed may be improved .

According to an embodiment of the present invention, the method further comprises calculating an add-on setting dis placement related control quantity based on the add-on set ting displacement using a first function, wherein adjusting the blade pitch angle based on the add-on setting displace ment comprises adjusting the blade pitch angle based on the add-on setting displacement related control quantity. The 'add-on setting displacement related control quantity' may be considered as a quantity which depends on or is a function of the add-on setting displacement. Thereby, the first function is utilized. The first function may be imple mented in different manners. In the first function it may be implemented to avoid adjusting the blade pitch angle unless the add-on setting displacement is relatively large, in order to reduce load of blade pitch angle adjustment equipment. By defining the first function high flexibility is provided for implementing the method for meeting particular application needs.

According to an embodiment of the present invention, the method further comprises calculating a 'rotational speed er ror related control quantity' based on the rotational speed error using a second function, wherein adjusting the blade pitch angle based on the rotational speed error comprises ad justing the blade pitch angle based on the 'rotational speed error related control quantity'.

The 'rotational speed error related control quantity' may be considered as a quantity which is dependent on or a function of the rotational speed error. The second function may allow a flexible implementation and may in particular allow to ad just the blade pitch angle only in cases wherein the rota tional speed error is relatively large or at least has such a value which cannot be decreased by adjusting the blade add-on (only). For example, for a relatively small rotational speed error the rotational speed error related control quantity may for example be zero. Only for a rotational speed error larger than for example a rotational speed error threshold, the ro tational speed error related control quantity may be differ ent from zero. Only in those cases the blade pitch angle may be adjusted, while for the rotational speed error being smaller than a speed error threshold there may be no blade pitch angle adjustment being performed. Herein rotational speed error may mean an absolute value (i.e. always positive or zero) of a difference (which may be positive or negative) between a reference rotational speed and the actual rotation al speed. Thereby, load and wear of blade pitch angle adjust ment equipment may be reduced.

According to an embodiment of the present invention, the method further comprises calculating at least 'one add-on setting related control quantity' based on the add-on setting using a third function including at least one add-on setting limit, calculating a 'rotational speed error and add-on re lated control quantity' based on the rotational speed error and the add-on setting related control quantity using the second function, wherein adjusting the blade pitch angle based on the rotational speed error comprises adjusting the blade pitch angle based on the 'rotational speed error and add-on related control quantity', wherein the add-on setting limit defines in particular a border setting beyond which the setting cannot be decreased or increased.

When the add-on setting related control quantity is calculat ed based on the add-on setting, also the capacity of the add on is considered, i.e. it is considered whether the add-on at all is capable of further adjustments for meeting the control objective. Thereby, the third function including at least one add-on setting limit, in particular a maximum add-on setting limit and a minimum add-on setting limit, is utilized. This capacity information can then be advantageously utilized for calculating the rotational speed error related control quan tity. Thus, it can be implemented to avoid adjusting the ro tor blade pitch angle not only when the rotational speed er ror is relatively small but also in the case when for example it turns out that the add-on has still sufficient capacity to perform the rotational speed control by its own. Furthermore, thereby it may be implemented to in fact adjust the rotor blade pitch angle if the speed error is relatively large and further if the capacity of the blade add-on is not sufficient to achieve the control objective. According to an embodiment of the present invention, the method includes supplying the rotational speed error to a speed-add-on controller which outputs a reference add-on set ting; supplying the reference add-on setting and a nominal add-on setting to a difference element to calculate the add on setting displacement; applying the first function to the add-on setting displacement to obtain the add-on setting dis placement related control quantity; applying the second func tion at least to the rotational speed error to obtain a rota tional speed error related control quantity; supplying the add-on setting displacement related control quantity and the rotational speed error related control quantity to a speed- pitch controller which outputs a reference blade pitch angle.

Thereby, a particular control scheme is provided for imple menting the method. Other control schemes may be derived and also being within the scope of the present invention which implement the functionality as described above in different embodiments. The add-on controller as well as the pitch con troller may be implemented for example as PI controller. All quantities may be physically realized as optical and/or elec trical and/or electronical and/or wireless signal and may be supplied using conductive wires, optical wires, as appropri ate. The method may in general be implemented in software and/or hardware. The first function and/or second function and/or third function may be implemented as software modules, for example. A blade pitch actuator may be supplied with the reference blade pitch angle causing the actuator to adjust the blade pitch angle of the rotor blade accordingly. An add on actuator may be supplied with the reference add-on setting which may result in the actuator adjusting or moving the add on (or at least an active surface thereof) in order to apply or adjust the add-on setting.

According to an embodiment of the present invention, the method further comprises applying the third function to the reference add-on setting to obtain the add-on setting related control quantity; applying the second function further to the add-on setting related control quantity and the rotational speed error to obtain a 'rotational speed error and add-on setting related control quantity'; applying the 'add-on set ting displacement related control quantity' and the 'rota tional speed error and add-on setting related control quanti ty' to a speed-pitch controller which outputs a reference blade pitch angle.

The first function acts on the add-on setting displacement, i.e. on the difference between a reference (or actual) add-on setting and the nominal add-on setting. The second function acts at least on the rotational speed error but may addition ally also act on the add-on setting related control quantity. This add-on setting related control quantity may capture whether the add-on still has adjustment capacity in order to fulfil control functions. The 'rotational speed error and add-on setting related control quantity' may capture the ro tational speed error as well as the add-on setting related control quantity. Therefore, this quantity is suitable to im plement a consideration of the rotational speed error as well as the add-on setting capacity. In this case, the pitch con troller may obtain or being supplied with as input two error values as input signal. The pitch controller (e.g. PI con troller) may be implemented as for example to operate on the sum (or weighted sum) of the error input. In other embodi ments, the two error inputs may be processed in parallel and the result of the two parallel branches may be added at the end.

According to an embodiment of the present invention, the first and/or the second function comprises at least one of a ID or 2D gain schedule function; a dead band function.

The gain schedule function and dead band function may provide different implementations of the different functions. There by, high flexibility is achieved. For a relatively simple im plementation a dead band function may be utilized for the first and also the second function. According to an embodiment of the present invention, the dead band function comprises a first linear section with positive slope up to a first error value, beyond the first error value a horizontal section up to a second error value, and beyond the second error value a second linear section with positive slope. The first error value as well as the second error val ue as well as the slope value may be defined or set depending on the particular application. More complex functions can be achieved using the 2D gain scheduling.

According to an embodiment of the present invention, the gain schedule function comprises a first non-linear section with positive slope (and/or being monotonically increasing, in cluding e.g. to be horizontal in the beginning for large er rors) up to a first error value, beyond the first error value a horizontal section up to a second error value, and beyond the second error value a second non-linear section with posi tive slope. Thereby, embodiments of the present invention are not restricted to linear responses to the different errors. Thereby, higher flexibility and better control performance may be achieved.

According to an embodiment of the present invention, the add on includes at least one of a one segment or multi-segment spoiler, in particular mounted along a longitudinal direction of the blade; a flap, in particular mounted at a tip end of the blade.

Possible add-ons may include:

1) Spoiler, e.g. having discrete setting, i.e. open or close, one- or multi-segmented.

2) Trailing edge flaps, continuous in operation (adjustable angle), e.g. one or more segments.

Other types of add-ons are possible. It should also be under stood that different kinds or manners of add-ons may simulta- neously be present at the rotor blade and all or each of these add-ons may be controlled in combination with the con trol of all other add-ons also in combination with or in de pendence of the rotor blade pitch control. In particular, em bodiments of the present invention may also envisage a stag gered control starting from add-ons which have the smallest impact on the aerodynamically property, followed by the con trol of add-ons having intermediate effect on the aerodynamic control or aerodynamic influence.

It should be understood that features, individually or in any combination, disclosed, described, explained or provided for a method of controlling a wind turbine by adjusting a blade pitch angle and at least one blade add-on of at least one wind turbine rotor blade are, individually or in any combina tion, also applicable to an arrangement for controlling a wind turbine by adjusting a blade pitch angle and at least one blade add-on of at least one wind turbine rotor blade ac cording to embodiments of the present invention and vice ver sa.

According to an embodiment of the present invention, it is provided an arrangement for controlling a wind turbine by ad justing a blade pitch angle and at least one blade add-on of at least one wind turbine rotor blade having the blade add on, the arrangement comprising a control module adapted to adjust a setting of the add-on to meet a control objective while temporarily maintaining a setting of the blade pitch angle.

Furthermore, it is provided a wind turbine, comprising at least one rotor blade having an add-on installed; and an ar rangement according to the preceding embodiment coupled to a blade pitch adjustment system and an add-on adjustment sys tem.

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 an arrangement for control ling a wind turbine by adjusting a blade pitch angle and at least one blade add-on of at least one wind turbine rotor blade having the blade add-on according to an embodiment of the present invention implementing a method of controlling a wind turbine according to an embodiment of the present inven tion;

Fig. 2 schematically illustrates an arrangement for control ling a wind turbine by adjusting a blade pitch angle and at least one blade add-on of at least one wind turbine rotor blade having the blade add-on according to an embodiment of the present invention implementing a method of controlling a wind turbine according to an embodiment of the present inven tion;

Figs. 3 and 4 illustrate examples of a 2D gain schedule as employed according to embodiments of the present invention;

Fig. 5 schematically illustrates an arrangement for control ling a wind turbine by adjusting a blade pitch angle and at least one blade add-on of at least one wind turbine rotor blade having the blade add-on according to an embodiment of the present invention implementing a method of controlling a wind turbine according to an embodiment of the present inven tion; Fig. 6 schematically illustrates an arrangement for control ling a wind turbine by adjusting a blade pitch angle and at least one blade add-on of at least one wind turbine rotor blade having the blade add-on according to an embodiment of the present invention implementing a method of controlling a wind turbine according to an embodiment of the present inven tion;

Figs. 7 to 9 illustrate possible implementations for deriving an error related quantity based on an error as utilized for example in embodiments of the present invention, in particu lar in a first function, a second function and/or a third function.

Detailed Description of Embodiments

The arrangement 100 schematically illustrated in Fig. 1 for controlling a wind turbine by adjusting a blade pitch angle and at least one blade add-on according to an embodiment of the present invention includes a control module 103 which is adapted to adjust a setting of the add-on to meet a control objective while maintaining a setting of the blade pitch an gle, thus being adapted to carry out a method according to an embodiment of the present invention.

Thereby, the setting of the add-on is adjusted by providing a reference add-on setting 105 to a not illustrated actuator which is configured to move the add-on (see e.g. Fig. 11 showing add-on 1065) for adjusting the setting. Furthermore, the arrangement 100 outputs a reference blade pitch angle 107. However, this reference rotor blade pitch angle 107 is maintained at least in a particular range of the speed error 109, as will be explained in detail below.

In the schemes illustrated in the figures, the control objec tive is to achieve and/or to track a reference rotational speed of the rotor at which the blade is mounted. The rota tional speed error 109 is thereby calculated and input to the arrangement 100 as being a difference between the reference rotational speed (for example the design speed of the wind turbine) and an actual rotational speed, for example measured or estimated and possibly filtered. Primarily, the blade add on may be adjusted by using the reference add-on setting 105, while the rotor blade pitch angle is maintained constant for relatively small rotational speed errors 109. The speed-add- on controller 110 determines the add-on setting 105 based on the rotational speed error 109.

However, the blade pitch angle may be adjusted based on the rotational speed error 109 and an add-on setting displacement 111, being a difference of the reference add-on setting 105 and a nominal add-on setting 113. The blade pitch angle may be adjusted only, if the rotational speed error is larger than a threshold and/or if the add-on setting displacement 111 is larger than a displacement threshold and/or if the add-on setting or the reference add-on setting 105 is closer to at least one add-on setting limit than at least one limit threshold (e.g. if the add-on setting is too close to the add-on setting limit, so that there is not enough adjustment capacity left). Thereby, unnecessarily adjusting the blade pitch angle may be avoided.

In Fig. 1 a first function block 115 is indicated which cal culates an add-on setting displacement related control quan tity 117 based on the add-on setting displacement 111.

The add-on setting displacement related control quantity 117 is input to a speed-pitch controller 119 and the reference blade pitch angle 107 is adjusted or calculated based on the add-on setting displacement related control quantity 117.

In Fig. 1 a second function module 121 is indicated which calculates based on the rotational speed error 109 a rota tional speed error related control quantity 123. The refer ence rotor blade pitch angle 107 is calculated based on the rotational speed error related control quantity 123. In par ticular, both, the add-on setting displacement related con- trol quantity 117 as well as the rotational speed error re lated quantity 123 are input to the speed-pitch controller 119 based on which the reference pitch angle 107 is derived.

The first function module 115 is implemented by a first gain schedule 125 which acts on the add-on setting displacement 111. The output of the gain schedule 125 is multiplied with the add-on setting displacement 111 using multiplication ele ment 120 to result in the add-on displacement related control quantity 117.

Furthermore, the second control module 121 is implemented us ing a second gain schedule 127 which acts on the rotational speed error 109. The output of the gain schedule 127 is mul tiplied with the rotational speed error 109 to result in the 'rotational speed error related control quantity' 123.

It should be understood that features or elements similar in structure and/or function in different embodiments are illus trated in the different figures with reference signs differ ing only in the first digit. A description of a particular element not in detail described with reference to a particu lar embodiment may be taken from the description of this cor responding element in another figure or embodiment.

According to an embodiment of the present invention, a speed control scheme is proposed utilizing both, active blade add ons and pitch actuators. Thus, at least two blade actuation systems are used working together to control the rotational speed of the rotor. Thereby, at least two controllers are utilized:

• A speed-add-on controller (for example a PID controller) that adjust a blade add-on reference to control rota tional speed, and

• a speed-pitch controller (for example a PID controller) that adjusts a pitch reference to control a combination of rotational speed and blade add-on position which gains schedules based on both quantities.

The control schemes illustrated in the figures may allow that blade add-ons have the highest priority for controlling the rotational speed whereas the pitch activity may be reduced and work primarily to get the operating point at a desired region. Pitch actuation is primarily used for handling large rotational speed fluctuations and for changing operating point.

In Figs. 1 and 2 gain scheduling is used to prioritize which actuator has priority for controlling rotational speed. The gain schedules (for example 125, 127) may have values between 0 and 1 and may be used to reduce pitch actuation while rota tional speed is maintained at its set point utilizing the blade add-ons.

The gain schedule 127 may ensure that the speed-pitch con troller 119 will not act on smaller speed errors that can be managed by the speed-add-on controller 110.

In Fig. 1 the rotational speed error 109 is provided as an input to a speed-add-on controller 110 which derives there from the reference add-on setting 105.

The add-on setting displacement 111 is applied to the first function 125. This first gain scheduling function 125 may en sure that the speed-pitch controller 119 only acts on an add on displacement 111 whenever the speed-add-on controller is a given distance from its nominal add-on setting 113. This fea ture may ensure that the pitch angle will on average keep the add-on at its nominal (desired) condition by (slowly) adapt ing to the new operating point.

The arrangement 200 illustrated in Fig. 2 has similarities to the arrangement 100 illustrated in Fig. 1. However, the ar rangement 200 additionally comprises a third function module 229. Therein, a add-on setting related control quantity 231 is calculated by the third function module 229 and is provid ed as an input to a 2-D gain schedule 233 together with the rotational speed error 209. As can be taken from Fig. 2, the reference blade pitch angle 207 is based on the rotational speed error 209 as well as on the add-on setting related con trol quantity 231.

As can be seen, the second control module 235 in Fig. 2 is similar to the second function module 121 illustrated in Fig. 1 and outputs the rotational speed error related control quantity 223 which is input to the speed-pitch controller 219 to derive the reference blade pitch angle 207 also based on the add-on setting displacement related control quantity 217 as output by the first control module 215 being similar to the first control module 115 illustrated in Fig. 1. Thus, in Fig. 2, the rotational speed error related quantity 223 is also derived based on the remaining capabilities of the add on from its limits.

In Fig. 2 the gain schedules 225, 233 may vary between 0 and 1 and are used to reduce pitch actuation while rotational speed is maintained at its set point utilizing the blade add on. In Fig. 2, the rotational speed error related control quantity 223 is obtained using a 2D lookup table or 2D gain schedule 233. The scheduling variables may be the speed error and some function of the add-on reference. This function can present the add-on reference as a percentage of the availa bility of add-on capacity. The add-on capacity is set by the minimum and maximum reference values.

In Fig. 2, the third function 229 is implemented by using a low pass filter 230, difference elements 232 and one division element 234. A minimum add-on reference 204 and a maximum add-on reference 206 are input to the difference elements 232 and to one of the difference elements 232 the output of the low pass filter 230 is provided. The output of the difference elements 232 is supplied to the division element 234. Using a gain element 236 an amplification of the output of the divi sion element 234 is performed. The resulting add-on setting related control quantity 231 is thus a function of the refer ence add-on setting 205, as well as the minimum and the maxi mum add-on setting 204, 206.

Two examples of the 2D gain schedules 338, 438 (graphs 343, 443) are presented in Figs. 3 and 4. Therein, the gain is in dicated on the ordinates 337, 437 which is plotted over the add-on reference capacity 339, 439 as well as the speed error 341, 441. Fig. 3 illustrates an example where for small speed errors and an add-on reference far away from its limits the gain is 0. By either a large speed error or an add-on refer ence close to its limits can make the gain scheduling in crease and become unity.

Fig. 4 illustrates an example, where the gain scheduling de pends on the sign of the speed error and on whether the add on is closer to its minimum or to its maximum limit. The 2D gain schedule lookup can also be set to vary across only one of its two dimensions: either speed error or a function of the add-on reference, making it effectively a ID gain sched ule.

According to an embodiment, the arrangements illustrated in Figs. 1 and 2 may be utilized to provide an example where the gain scheduling is used to prioritize which actuator has pri ority for control the rotational speed, giving that the add ons are operated in an on/off manner. This may for example be applied when the add-on is formed by several segments (for example 4 to 10 or 12) which may be open or closed or on and off (not to be controlled continuously).

Given the number of blade add-on segments N, the maximum lim it for the blade add-on reference may be defined in percent age of the available segments as (in %)

Rmax (N - N failed)/N * 100, where N_failed is the number of failed segments. In this ex ample, the lower reference limit is left unchanged at Rmin=0%. Rmin may relate to 204 and Rmax may relate to 206 in Fig. 2.

One or more active add-on devices and concepts can be used. One example is the use of segmented add-ons where a set of elements can independently cause an aerodynamic stall at lo calized sections of the rotor blade. Active flaps may be an other add-on device.

The speed control concept explained herein, may have multiple applications, since the speed control is a discipline in mul tiple applications of wind turbine control. This may include, but not be restricted to:

• Use of active blade add-ons for speed control during curtailed operation. This may reduce pitch activity. Thereby, it may be that requirements to speed tracking are relaxed for such operation.

• Use active blade add-ons for speed control during active idling. During active idling, it may be sufficient to control the speed inside a rotational speed range (as opposed to a single set point value) and control may happen via active blade add-ons alone, saving wear and energy consumption for the pitch system, or use the pitch system only rarely to have partial sav ings/reductions .

• Use active blade add-ons for speed control during reso nance avoidance. There are several speed regions where operation is unintended, for example if the tower fre quency and the IP or 3P rotation frequency collide.

Here, active blade add-ons can be applied to force oper ation out of the speed regions. • Use active blade add-ons for speed control during high wind speed operation. This may reduce pitch activity. It may be that requirements for speed tracking is relaxed for such operation, at least if speed support is reduced for high winds.

• Use active blade add-ons for speed control during self- sustained operation. It may be desired to control speed by active blade add-ons to save wear and power consump tion of a pitch system.

Several of the following advantages or technical features may be achieved: Costs may be reduced because blade pitch bearing and (hydraulic or electrical) pitch system loading may be re duced due to reduced pitched reference excitation via gain scheduling that excludes control errors that can be handled by add-ons. Alternatively, this may enable larger turbines with similar hardware for pitch bearings and pitch system.

Performance in power production may be maintained or improved because add-ons acting on speed error may be used for main taining or improving speed regulation performance resulting in fewer drops below nominal speed and/or leading to fewer drops below nominal power. Control of the add-on according to embodiments of the present invention may also be performed during startup dependent on wind speed and power reference.

Figs. 5 and 6 illustrate further embodiments of arrangements 500 and 600 for controlling according to embodiments of the present invention, wherein the control modules 515, 521 do not use gain schedules but use a dead zone function, in par ticular a first dead zone function 526 and a second dead zone function 528. However, these first function 515 and second function 521 have similar effects as the functions 115 and 121 illustrated in Fig. 1.

In Fig. 6, the third function 629 also comprises a low pass filter 630 and difference elements 632. However, the outputs of the difference elements are provided to gain elements 614, 616 which provide a definition in the positive range of the dead zone function 628 and a definition in the negative range of the dead zone function, respectively. Thereby, the width of this dead zone element 628 may be set by the difference between the add-on reference and add-on minimum and maximum limits, which represents the remaining ability of the add-ons to act to regulate speed in each direction. Thus, the outputs of the third function module 629 may define from which error value on at the positive error range the output of the dead zone 628 is different from 0 and from which error value on the output in the negative error range is different from 0.

Alternatively or additionally for example the slope for posi tive rotational speed error may be differently adjusted as the slope of the dead zone module 628 in the negative rota tional speed error range. Thereby, more flexibility is achieved .

Figs. 7, 8 and 9 illustrate different implementations of first or second function modules 721, 821, 921, which can for example be utilized in any of the afore-described embodiments as first function module and/or second function module. The schedule or gain schedule 725 of the first function module 721 illustrated in Fig. 7 is applied to the rotational speed error 709 and the result is multiplied with the rotational speed error 709 to result in the speed error related control signal 723.

In Fig. 8 the rotational speed error 809 is applied to the gain schedule 825 to result in the rotational speed related control signal 823.

In Fig. 9 the rotational speed error 909 is applied to the dead zone element or function 928 to result in the rotational speed related control quantity 923. Fig. 10 illustrates a coordinate system having an abscissa 1050 indicating an error and an ordinate 1052 indicating an output of a gain schedule module or a dead zone module such as those used and illustrated in the afore-mentioned figures. The curve 1051 illustrates an example of a dead zone and the curve 1053 illustrates an example gain scheduling.

The dead zone curve 1051 comprises a first linear section 1051a with a positive slope up to a first error value 1055, comprises a horizontal section 1051b beyond the first error value and up to a second error value 1057 and further com prises beyond the second error value 1057 a second linear section 1051c.

The gain scheduling curve 1053 comprises a first non-linear section 1053a up to the first error value 1055, comprising a horizontal section 1053b beyond the first error value 1055 and up to a second error value 1057 and comprises a second non-linear section 1053c beyond the second error value 1057. Outside the first error value and the second error value the gain scheduling function 1053 may comprise linear sections.

Fig. 11 schematically illustrates a wind turbine 1060 com prising a rotor 1061 having mounted thereon plural rotor blades 1063 wherein at least one has an adjustable add-on 1065, for example comprising plural segments 1067 of a spoil er. The wind turbine 1060 comprises further an arrangement 1000 which may be configured as the arrangements 100, 200 or 500 or 600 illustrated in Figs. 1, 2, 5 or 6. The arrangement 1000 controls the setting of the add-on 1065 as well as the rotor blade pitch angle by using a pitching system 1069, wherein the rotational speed 1062 as measured by a sensor 1064 (and e.g. other input values) is received. The rotor 1061 is harbored inside a nacelle 1071 which is mounted on top of a wind turbine tower 1073.

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.