Patent Description:
Active tower damping reduces the tower oscillations by applying 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 oscillations 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 pitching is required for damping the tower oscillations in addition 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.

<CIT> discloses flaps which are mounted to a blade of a wind turbine rotor. The flaps change an airflow to regulate a rotation speed of the rotor. <CIT> is a further example of prior art.

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 independent claims. The present invention is further developed as set forth in the dependent claims.

According to a first aspect of the invention, a method according to claim <NUM> of damping front and backward movements of a tower of a wind turbine is provided.

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 oscillations. 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.

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 nacelle; 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 frequency component and supplying the actuating signals to the actuators to actuate the corresponding add-on member.

The step b) further comprises the following substeps: compensating a phase of the time signal 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 applying an offset to the active add-on members. The damping signal to the actuators can be a function of the tower acceleration 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 apply a thrust change that will dampen the tower oscillation. For example, the tower oscillations are triggered when a rotating blade passes the tower. Thereby, the air pressure between 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 acceleration of the tower (or the nacelle), and is conducted in the following steps: <NUM>) measuring the forward and backward acceleration of nacelle or tower top, <NUM>) filtering the acceleration signal to ensure a) that relevant frequency component(s) is/are passed through, otherwise dampened; and b) that a phase at the dominant movement (first tower eigen frequency) is taken into account to ensure that the tower oscillations 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 ensure 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 filter 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 aerodynamics. Obtaining the correct phase can depend on the transfer function between measured acceleration and rotor thrust change, including possible communication delays, actuator 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 acceleration 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 relevant 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 additional activation of actuators is not desired for some operating 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.

Each blade is movable to alter a pitch angle thereof; and the pitch angle is determined by taking the actuating 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 controllers, 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 combines 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 according to claim <NUM> for damping front and backward movements of a tower of a wind turbine is provided.

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 rotor 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 actuating signal of the corresponding actuator into account.

<FIG> shows a wind turbine blade <NUM> 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 rotate 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 <NUM>. Each blade <NUM> has an active add-on member <NUM> which is actuated by an actuator to alter aerodynamic properties of the blade <NUM>.

Via the blades <NUM>, 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 <NUM> is actuated by the corresponding actuator to alter the aerodynamic properties of the blade <NUM> 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 <NUM>, a thrust force from the rotor to the tower is appropriately changed to counteract against the front and backward movements of the tower.

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

<FIG> shows the same add-on member <NUM> in an activated position, where the add-on member <NUM> is turned to a maximum by the actuator so that the stalling effect is maximum.

According to the present invention, the add-on member <NUM> is not necessarily to be formed as a spoiler. The add-on member <NUM> can have any other configuration which is able to alter the aerodynamic properties of the blade <NUM>.

<FIG> 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 <NUM> designates a rotor speed reference; reference sign <NUM> designates a speed trim stall controller; reference sign <NUM> designates a trim stall system; reference sign <NUM> designates a the rotor speed; reference sign <NUM> designates a trim stall reference; reference sign <NUM> designates a trim stall pitch controller; reference sign <NUM> designates a pitch system; reference sign <NUM> designates a tower acceleration; and reference sign <NUM> designates a tower trim stall controller.

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

In the tower trim stall controller <NUM>, the time signal is filtered to extract at least one frequency component. The filtering 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 filtered frequency component can be compensated. To compensate for possible communication delays, actuator dynamics, actuator 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 frequency component. The individual actuating signals are then supplied to the actuators of the add-on member <NUM> of each blade <NUM>. The gains can be determined such that the activation signals 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 frequency 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 <NUM> at a certain operating point. The variable value can apply a damping only at a selected operating point.

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

In the first embodiment, a difference between the rotor speed reference <NUM> and the rotor speed <NUM> is input in the speed trim stall controller <NUM>. The output of the speed trim stall controller <NUM> is input in the trim stall system <NUM> together with the actuating signal which is output from the trim stall controller <NUM> as described above. The actuators of the add-on members <NUM> of each blade <NUM> therefore also take the rotor speed into account.

Further, a difference between an output from the speed trim stall controller <NUM> and the trim stall reference <NUM> is input into the trim stall pitch controller <NUM>. An output of the trim stall pitch controller <NUM> is input in the pitch system <NUM>. The pitch system <NUM> comprises a pitch actuator to change a pitch angle of the corresponding blade <NUM>. The pitch angle is an angle of the blade <NUM> which is measured about the longitudinal axis of the blade <NUM>.

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 <NUM>, wherein the pitch system <NUM> is not involved herein. This is particularly an advantage where large and heavy blades <NUM> are used because the pitch system <NUM> is focused mainly on the pitching and not overstrained by the trim stall work as an additional task. As a result, pitch activity and pitch bearing damages can be avoided.

<FIG> shows a relationship between pitch and trim stall activations, and <FIG> shows a relation between the thrust coefficient (Ct) and the trim stall activation, where the pitch angle will vary according to <FIG>.

For very 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 and pitch controllers, the activation of trim stall system <NUM> 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> 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> are designated by the same reference signs. In the second embodiment, a trim stall reference and a pitch reference are generally calculated as a function of the tower/nacelle accelerations.

In more detail, a front and backward acceleration of the tower is measured to obtain the tower acceleration <NUM>. The tower acceleration <NUM> is input in the tower trim stall controller <NUM>.

In the tower trim stall controller <NUM>, at least one frequency component from the measured acceleration is filtered, and a phase of the at least one filtered frequency component is determined. 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 <NUM>.

The actuating signal is supplied from the trim stall controller <NUM> to the trim stall system <NUM>.

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

The trim stall system <NUM> comprises the active add-on members <NUM> and the actuators of each blade <NUM>. Each actuator is operated based on the supplied actuating signal so that the active add-on member <NUM> is actuated by the actuator to alter the aerodynamic properties of the corresponding blade <NUM>. Further, a difference between an output from the speed trim stall controller <NUM> and the trim stall reference <NUM> is input into the trim stall pitch controller <NUM>. An output of the trim stall pitch controller <NUM> is input in the pitch system <NUM>. The pitch system <NUM> comprises a pitch actuator to change a pitch angle of the corresponding blade <NUM>. The pitch angle is an angle of the blade <NUM> which is measured about the longitudinal axis of the blade.

In the second embodiment, the measured tower acceleration <NUM> is additionally input in a tower pitch controller <NUM>. The output of the tower pitch controller <NUM> is input in the pitch system <NUM> together with the output of the above described trim stall pitch controller <NUM>.

In the second embodiment, the tower acceleration <NUM> is taken into account into the pitch reference so that the pitch system <NUM> assists the dampening of the front and backward movements of the tower.

<FIG> 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 embodiment of <FIG> are designated by the same reference signs. The third embodiment is an alternative of the second embodiment. 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 <NUM>. The tower acceleration <NUM> is input in the tower pitch controller <NUM>. An output of the tower pitch controller <NUM> is input in the tower trim stall controller <NUM>.

In the tower trim stall controller <NUM>, at least one frequency component from the measured acceleration is filtered, and a phase of the at least one filtered frequency component is determined. 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 filtering process and/or the application of the gain is performed considering the output of the tower pitch controller <NUM>.

The actuating signal is supplied from the trim stall controller <NUM> to the trim stall system <NUM>. The trim stall system <NUM> comprises the active add-on members <NUM> and the actuators of each blade <NUM>. Each actuator is operated based on the supplied actuating signal so that the active add-on member <NUM> is actuated by the actuator to alter the aerodynamic properties of the corresponding blade <NUM>.

In the third embodiment, a difference between the rotor speed reference <NUM> and the rotor speed <NUM> is input in the speed trim stall controller <NUM>. The output of the speed trim stall controller <NUM> is input in the trim stall system <NUM> together with the actuating signal which is output from the trim stall controller <NUM> as described above. The actuators of the add-on members <NUM> of each blade <NUM> therefore also take the rotor speed into account.

The output of the tower pitch controller <NUM> is input in the pitch system <NUM> together with the output of the trim stall pitch controller <NUM> as described above.

In the third embodiment, the trim stall reference is also determined based on the pitch control.

Claim 1:
A method of damping front and backward movements of a tower of a wind turbine, 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 backward movements of the tower occur, and the rotor has a plurality of blades (<NUM>), wherein each blade (<NUM>) is movable to alter a pitch angle thereof and has at least one corresponding active add-on member (<NUM>) which is actuated by a corresponding actuator to alter aerodynamic properties of the blade (<NUM>);
wherein each add-on member (<NUM>) is actuated by the corresponding actuator to alter the aerodynamic properties of the blade (<NUM>) in a manner that the rotor is configured to damp the front and backward movements of the tower of the wind turbine, wherein the method comprises
a) measuring a time signal which represents a front and backward acceleration, velocity or position of the tower or the nacelle;
b) filtering the time signal to extract at least one frequency component, wherein step b) further comprises the following sub steps:
compensating a phase of the time signal 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; and
c) generating an actuating signal for each actuator based on the at least one extracted frequency component and supplying the actuating signals to the actuators to actuate the corresponding add-on member (<NUM>); wherein
the pitch angle is determined by taking the actuating signal of the corresponding actuator into account.