Device and method of damping front and backward movements of a tower of a wind turbine

Field of invention

The present invention relates to a device and a method of damping front and backward movements of a tower of a wind turbine .

Active tower damping reduces the tower oscillations by apply ing appropriate pitch angle changes. The pitch angle is an angle which is measured about a longitudinal axis of each blade. An appropriate change in the pitch angle changes the aerodynamic properties of the blade so that the tower oscil lations can be dampened. This reduces the bending loads of the tower and the tower base. However, pitch activity and pitch bearing damages will increase, since additional pitch ing is required for damping the tower oscillations in addi tion to the ordinary pitching operation. If a pitch system capacity and/or a pitch bearing capacity are an issue for a certain wind turbine designs, particularly for large and heavy blades or other constraints, the tower damping can hardly be achieved by the pitch system.

EP 1 320 680 A1 discloses flaps which are mounted to a blade of a wind turbine rotor. The flaps change an airflow to regu late a rotation speed of the rotor.

Summary of the Invention

There may be a need for a device and a method of damping front and backward movements of a tower of a wind turbine which take the rotor and blade constraints into account. This need may be met by the subject matters according to the inde pendent claims. The present invention is further developed as set forth in the dependent claims. According to a first aspect of the invention, a method of damping front and backward movements of a tower of a wind turbine is provided, wherein the wind turbine comprises the tower and a rotor, the rotor being mounted at the top of the tower to rotate about a rotational axis in which the front and back-ward movements of the tower occur, and the rotor has a plurality of blades, wherein each blade has at least one corresponding active add-on member which is actuated by a corresponding actuator to alter aerodynamic properties of the blade. Each add-on member is actuated by the corresponding actuator to alter the aerodynamic properties of the blade in a manner that the rotor is configured to damp the front and backward movements of the tower of the wind turbine.

Advantageously, a pitch system is basically not necessarily involved in the damping so that the constraints of the pitch system or the blade are not relevant anymore for the in ventive method. The active add-on members can reduce struc tural loads (primarily tower and foundation loads) , and the pitch system (e.g. hydraulics) is not stressed. Pitch bearing damages can thus be avoided.

The add-on members will lower the thrust of the rotor when they are activated (similar to pitching the blade towards feather will lower the thrust in the traditional set up) . Hence, if the add-on members are activated when the tower is to move backwards, thrust can be reduced, and the tower will not sway that much backwards, which will reduce the oscilla tions. Similarly, when the tower is to be pushed forwards, the add-on members can be activated to increase the thrust to fight against the movement of the tower oscillation.

Preferably, the method comprises the following steps: a) measuring a time signal which represents a front and backward acceleration, velocity or position of the tower or the na celle; b) filtering the time signal to extract at least one frequency component; and c) generating an actuating signal for each actuator based on the at least one extracted fre quency component and supplying the actuating signals to the actuators to actuate the corresponding add-on member.

Preferably, the step b) can further comprises at least one of the following substeps: compensating a phase of the time sig nal for the at least one filtered frequency component, and applying gains to obtain individual actuating signals for each actuator to act on the at least one frequency component.

The active tower damping function reduces loads on the tower base resulting from the front and backward movements by ap plying an offset to the active add-on members. The damping signal to the actuators can be a function of the tower accel eration signal. The damping signal contains the frequency content of the dominant tower movement (the first tower eigen mode) .

The damping signal can have an optimal phase at the first tower eigen mode such that the active add-on members will ap ply a thrust change that will dampen the tower oscillation. For example, the tower oscillations are triggered when a ro tating blade passes the tower. Thereby, the air pressure be tween that blade and the tower is suddenly changed so that the tower oscillations are triggered. Since the phase of the tower oscillation is determined, the actuator of each blade is actuated in a correct timing.

The offset of the add-on members (since this will be added to existing controller outputs for active add-on members) can be calculated based on the measured forward and backward accel eration of the tower (or the nacelle) , and is conducted in the following steps: 1) measuring the forward and backward acceleration of nacelle or tower top, 2) filtering the accel eration signal to ensure a) that relevant frequency compo nent (s) is/are passed through, otherwise dampened; and b) that a phase at the dominant movement (first tower eigen fre- quency) is taken into account to ensure that the tower oscil lations are correctly dampened by respective add-on members.

More preferred, step of applying gains comprises a sub step of limiting the activation signals within upper and/or lower bounds. Thereby, the actuating signal can be saturated to en sure that the offset is within desired limits that respect the capacity of the add-on members and the desired usage range .

More preferred, step b) is performed using at least one of a low-pass filter and a bandpass filter. The low-pass filter can act to change the phase of the signal (tuned to adjust it sufficiently at the tower frequency) , while the bandpass fil ter can pass through only the first tower eigen frequency (i.e. the dominant movement) .

More preferred, the step of compensating a phase of the time signal is performed using a transfer function between the measured acceleration and a rotor thrust change to compensate for possible communication delays, actuator dynamics and aer odynamics. Obtaining the correct phase can depend on the transfer function between measured acceleration and rotor thrust change, including possible communication delays, actu ator dynamics, actuator delays, sensor delays, system delays, aerodynamics, etc.

More preferred, the gains in the step of applying gains have fixed values. The gains are applied to convert the accelera tion signal into an appropriate activation of the active add on member. Alternatively, the gains in the step of applying gains have variable values which are determined based on a sensitivity of the corresponding active add-on member at a certain operating point, and/or which apply a damping only at a selected operating point. The gain can be scheduled by rel evant operational parameters to include sensitivity of the active add-on at a certain operating point, or the gain can be scheduled to apply the damping only at selected operating points, e.g. only in certain operating regions, if an addi tional activation of actuators is not desired for some oper ating points.

Preferably, each blade is movable to alter a pitch angle thereof; and the active add-on member of each blade is moved by the corresponding actuator based on the actuating signal while a pitch angle of the same blade is kept unchanged.

Thereby, the pitch system is not stressed at all.

Preferably, each blade is movable to alter a pitch angle thereof; and the active add-on member of each blade is moved by the corresponding actuator based on the actuating signal which takes a pitch angle of the same blade into account. Thereby, the pitch system is not necessarily to be switched- off, but it is not stressed by the dampening operation of the add-on members.

Preferably, each blade is movable to alter a pitch angle thereof; and the pitch angle is determined by taking the ac tuating signal of the corresponding actuator into account.

For example, at low tower frequencies or floating foundations (having a low frequency as well) , where the system frequency is so low that it will interfere with the speed/pitch con trollers, the activation of the add-on members could have an opposite impact, as the resulting pitch angle will change the thrust with the opposite sign. However, this embodiment com bines the pitch angle and add-on member activations, where the power and speed will be the same, but the rotor thrust will vary.

According to a second aspect of the invention, a device for damping front and backward movements of a tower of a wind turbine is provided, wherein the wind turbine comprises the tower and a rotor, the rotor being mounted at the top of the tower to rotate about a rotational axis in which the front and back-ward movements of the tower occur, and the rotor has a plurality of blades, wherein each blade has at least one corresponding active add-on member which is actuated by a corresponding actuator to alter aerodynamic properties of the blade. Each add-on member is actuated by the corresponding actuator to alter the aerodynamic properties of the blade in a manner that the rotor is configured to damp the front and backward movements of the tower of the wind turbine.

Preferably, the device further comprises: a measuring device configured to measure a time signal which represents a front and backward acceleration, velocity or position of the tower or the nacelle, a filtering device configured to filter the time signal to extract at least one frequency component, and a generating and supplying device configured to generate an actuating signal for each actuator based on the at least one extracted frequency component and to supply the actuating signals to the actuators to actuate the corresponding add-on member .

Preferably, the filtering device further comprises at least one of a compensating device configured to compensate a phase of the time signal for the at least one filtered frequency component, and an applying device configured to apply gains to obtain individual actuating signals for each actuator to act on the at least one frequency component.

More preferred, the applying device is configured to limit the activation signals within upper and/or lower bounds.

More preferred, the filtering device comprises at least one of a low-pass filter and a bandpass filter.

More preferred, the determining device is configured to use a transfer function between the measured acceleration and a ro tor thrust change to compensate for possible communication delays, actuator dynamics, actuator delays, sensor delays, system delays, and aerodynamics. More preferred, the gains have fixed values. Alternatively, the gains have variable values which are determined based on a sensitivity of the corresponding active add-on member at a certain operating point, and/or which applies a damping only at a selected operating point.

Preferably, each blade is movable to alter a pitch angle thereof; and the active add-on member of each blade is moved by the corresponding actuator based on the actuating signal while a pitch angle of the same blade is kept unchanged.

Preferably, each blade is movable to alter a pitch angle thereof; and the active add-on member of each blade is moved by the corresponding actuator based on the actuating signal which takes a pitch angle of the same blade into account.

Preferably, each blade is movable to alter a pitch angle thereof; and the pitch angle is determined by taking the ac tuating signal of the corresponding actuator into account.

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 apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other noti fied, in addition to any combination of features belonging to one type of subject matter also any combination between fea tures relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application.

Brief Description of the Drawings

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.

Fig. 1 shows a wind turbine blade having an add-on member;

Fig. 2 shows the same add-on member in an activated position, where the add-on member is turned to maximum stalling effect ;

Fig. 3 shows an implementation of the method of damping front and backward movements of a tower of a wind turbine according to a first embodiment;

Fig. 4 shows a relationship between pitch and trim stall ac tivations ;

Fig. 5 shows a relation between the thrust coefficient (Ct) and the trim stall activation, where the pitch angle will vary according to Fig. 4;

Fig. 6 shows an implementation of the method of damping front and backward movements of a tower of a wind turbine according to a second embodiment; and

Fig. 7 shows an implementation of the method of damping front and backward movements of a tower of a wind turbine according to a third embodiment.

Detailed Description

The illustrations in the drawings are schematically. It is noted that in different figures, similar or identical ele ments are provided with the same reference signs.

Fig . 1 shows a wind turbine blade 15 of a wind turbine (not shown) . The wind turbine comprises a tower and a rotor, wherein the rotor is mounted at the top of the tower to ro tate about a rotational axis. In more detail, the rotor is mounted to a hub which in turn is mounted to a nacelle. The nacelle is mounted to the tower. The rotor has a plurality of the blades 15. Each blade 15 has an active add-on member 17 which is actuated by an actuator to alter aerodynamic proper ties of the blade 15.

Via the blades 15, the rotor applies a thrust force to the tower so that front and backward movements of the tower and a nacelle of the tower occur.

Each add-on member 17 is actuated by the corresponding actua tor to alter the aerodynamic properties of the blade 15 in a manner that the rotor is configured to damp the front and backward movements of the tower of the wind turbine. That is, by changing the aerodynamic properties of the blades 15, a thrust force from the rotor to the tower is appropriately changed to counteract against the front and backward move ments of the tower.

The add-on member 17 is designed as a spoiler. The spoiler 17 is here arranged near the front edge of the blade 15, but can also be arranged near the back edge of the blade 15. The add on member 17 is accommodated in a recess 16 in the blade 15 and can turn about a hinge 18 by activation of the actuator. In Fig. 1, the spoiler 17 is shown in its normal deactivated position, where no spoiler effect and no stall is desired.

Fig . 2 shows the same add-on member 17 in an activated posi tion, where the add-on member 17 is turned to a maximum by the actuator so that the stalling effect is maximum.

According to the present invention, the add-on member 17 is not necessarily to be formed as a spoiler. The add-on member 17 can have any other configuration which is able to alter the aerodynamic properties of the blade 15. Fig. 3 shows an implementation of the method of damping front and backward movements of a tower of a wind turbine according to a first embodiment.

Reference 1 designates a rotor speed reference; reference sign 2 designates a speed trim stall controller; reference sign 3 designates a trim stall system; reference sign 4 des ignates a the rotor speed; reference sign 5 designates a trim stall reference; reference sign 6 designates a trim stall pitch controller; reference sign 7 designates a pitch system; reference sign 8 designates a tower acceleration; and refer ence sign 9 designates a tower trim stall controller.

A front and backward acceleration of the tower is measured to obtain the tower acceleration 8. The tower acceleration 8 is input in the tower trim stall controller 9. Alternatively, a front and backward velocity of the tower or even a position of the tower in the front and backward direction can be meas ured instead of the acceleration.

In the tower trim stall controller 9, the time signal is fil tered to extract at least one frequency component. The fil tering can be performed by using at least one of a low-pass filter and a bandpass filter. The filtering is achieved by a bandpass filter and then optionally by a low pass filter to compensate the phase, and as a secondary objective to further attenuate noise. Optionally, a time delay and/or a lead-lag filter can be further provided. The used filter works on the time series (for example for the measured tower or nacelle acceleration signal) and is designed to have a characteristic which aim to output a particular frequency (the tower mode) such that it has a high amplitude (level) and the correct phase (time lead/lag) for damping purposes.

Thereby, a phase of the time signal for the at least one fil tered frequency component can be compensated. To compensate for possible communication delays, actuator dynamics, actua tor delays, sensor delays, system delays and aerodynamics, etc., the phase can be compensated for by use a transfer function between the measured acceleration and a rotor thrust change .

Thereafter, gains are applied to obtain individual actuating signals for each actuator to act on the at least one frequen cy component. The individual actuating signals are then sup plied to the actuators of the add-on member 17 of each blade 15. The gains can be determined such that the activation sig nals are limited within upper and/or lower bounds. The gains are applied to the time signal, so it will in principle be applied to all frequencies present in the signal. However, the filter is usually set such that mainly the tower frequen cy will exist in the signal.

The gains can either have a fixed value or a variable value. The variable value can be determined based on a sensitivity of the active add-on member 17 at a certain operating point. The variable value can apply a damping only at a selected op erating point.

The thus generated actuating signals are supplied from the trim stall controller 9 to the trim stall system 3. The trim stall system 3 comprises the active add-on members 17 and the actuators of each blade 15. Each actuator is operated based on the supplied, associated actuating signal so that the ac tive add-on members 17 are actuated by the actuators to alter the aerodynamic properties of the corresponding blades 15.

In the first embodiment, a difference between the rotor speed reference 1 and the rotor speed 4 is input in the speed trim stall controller 2. The output of the speed trim stall con troller 2 is input in the trim stall system 3 together with the actuating signal which is output from the trim stall con troller 9 as described above. The actuators of the add-on members 17 of each blade 15 therefore also take the rotor speed into account. Further, a difference between an output from the speed trim stall controller 2 and the trim stall reference 5 is input into the trim stall pitch controller 6. An output of the trim stall pitch controller 6 is input in the pitch system 7. The pitch system 7 comprises a pitch actuator to change a pitch angle of the corresponding blade 15. The pitch angle is an angle of the blade 15 which is measured about the longitudi nal axis of the blade 15.

In the present invention, the front and backward movements of a tower of a wind turbine are preferably exclusively dampened by the separate trim stall system 3, wherein the pitch system 7 is not involved herein. This is particularly an advantage where large and heavy blades 15 are used because the pitch system 7 is focused mainly on the pitching and not over strained by the trim stall work as an additional task. As a result, pitch activity and pitch bearing damages can be avoided .

Fig. 4 shows a relationship between pitch and trim stall ac tivations, and Fig. 5 shows a relation between the thrust co efficient (Ct) and the trim stall activation, where the pitch angle will vary according to Fig. 4.

For very low tower frequencies or floating foundations (hav ing a low frequency as well) , where the system frequency is so low that it will interfere with the speed and pitch con trollers, the activation of trim stall system 3 will have the opposite impact, as the resulting pitch angle will change the thrust with the opposite sign.

Hence, it shall be exploited that there is a combination of a pitch angle activation and a trim stall activation, where the power and speed will be the same, but the rotor thrust will vary .

Fig. 6 shows a corresponding implementation of the method of damping front and backward movements of a tower of a wind turbine according to a second embodiment which is suitable for these very low tower frequencies or floating foundations. The same elements of the first embodiment of Fig. 3 are des ignated by the same reference signs. In the second embodi ment, a trim stall reference and a pitch reference are gener ally calculated as a function of the tower/nacelle accelera tions .

In more detail, a front and backward acceleration of the tow er is measured to obtain the tower acceleration 8. The tower acceleration 8 is input in the tower trim stall controller 9.

In the tower trim stall controller 9, at least one frequency component from the measured acceleration is filtered, and a phase of the at least one filtered frequency component is de termined. Thereafter, gains are applied on the at least one filtered frequency component having the determined phase to obtain actuating signals for each add-on member 17.

The actuating signal is supplied from the trim stall control ler 9 to the trim stall system 3.

In the second embodiment, a difference between the rotor speed reference 1 and the rotor speed 4 is input in the speed trim stall controller 2. The output of the speed trim stall controller 2 is input in the trim stall system 3 together with the actuating signal which is output from the trim stall controller 9 as described above. The actuators of the add-on members 17 of each blade 15 therefore also take the rotor speed into account.

The trim stall system 3 comprises the active add-on members 17 and the actuators of each blade 15. Each actuator is oper ated based on the supplied actuating signal so that the ac tive add-on member 17 is actuated by the actuator to alter the aerodynamic properties of the corresponding blade 15. Further, a difference between an output from the speed trim stall controller 2 and the trim stall reference 5 is input into the trim stall pitch controller 6. An output of the trim stall pitch controller 6 is input in the pitch system 7. The pitch system 7 comprises a pitch actuator to change a pitch angle of the corresponding blade 15. The pitch angle is an angle of the blade 15 which is measured about the longitudi nal axis of the blade.

In the second embodiment, the measured tower acceleration 8 is additionally input in a tower pitch controller 10. The output of the tower pitch controller 10 is input in the pitch system 7 together with the output of the above described trim stall pitch controller 6.

In the second embodiment, the tower acceleration 8 is taken into account into the pitch reference so that the pitch sys tem 7 assists the dampening of the front and backward move ments of the tower.

Fig . 7 shows an implementation of the method of damping front and backward movements of a tower of a wind turbine according to a third embodiment. The same elements of the second embod iment of Fig. 6 are designated by the same reference signs. The third embodiment is an alternative of the second embodi ment. In the third embodiment, the trim stall part of the damping is calculated as a function of the pitch part of the damping .

A front and backward acceleration of the tower is measured to obtain the tower acceleration 8. The tower acceleration 8 is input in the tower pitch controller 10. An output of the tow er pitch controller 10 is input in the tower trim stall con troller 9.

In the tower trim stall controller 9, at least one frequency component from the measured acceleration is filtered, and a phase of the at least one filtered frequency component is de- termined. Thereafter, a gain is applied on the at least one filtered frequency component having the determined phase to obtain an actuating signal. In the third embodiment, the fil tering process and/or the application of the gain is per formed considering the output of the tower pitch controller 10.

The actuating signal is supplied from the trim stall control ler 9 to the trim stall system 3. The trim stall system 3 comprises the active add-on members 17 and the actuators of each blade 15. Each actuator is operated based on the sup plied actuating signal so that the active add-on member 17 is actuated by the actuator to alter the aerodynamic properties of the corresponding blade 15.

In the third embodiment, a difference between the rotor speed reference 1 and the rotor speed 4 is input in the speed trim stall controller 2. The output of the speed trim stall con troller 2 is input in the trim stall system 3 together with the actuating signal which is output from the trim stall con troller 9 as described above. The actuators of the add-on members 17 of each blade 15 therefore also take the rotor speed into account.

Further, a difference between an output from the speed trim stall controller 2 and the trim stall reference 5 is input into the trim stall pitch controller 6. An output of the trim stall pitch controller 6 is input in the pitch system 7. The pitch system 7 comprises a pitch actuator to change a pitch angle of the corresponding blade 15. The pitch angle is an angle of the blade 15 which is measured about the longitudi nal axis of the blade 15.

The output of the tower pitch controller 10 is input in the pitch system 7 together with the output of the trim stall pitch controller 6 as described above. In the third embodiment, the trim stall reference is also de termined based on the pitch control.

It should be noted that the term "comprising" does not ex- elude other elements or steps and "a" or "an" does not ex clude 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.