REDUCTION OF EDGEWISE VIBRATIONS USING BLADE LOAD SIGNAL

Abstract

The present invention relates to control of a wind turbine to reduce structural loading due to vibrations of the blades along the edgewise direction. A rotor control system for actuating pitch of pitch-adjustable rotor blades of a wind turbine is disclosed. Pitch modification signals are determined based on edgewise load signals for each of the rotor blades. The edgewise load signal are coordinated transformed and input into a primary whirling controller unit to provide whirling signal components which can be used for determining the pitch modification signals.

Claims

1. A rotor control system for actuating pitch of pitch adjustable rotor blades of a wind turbine, the rotor control system comprising a pitch actuation unit for determining pitch modification signals to be applied to a pitch actuator for actuating the pitch of the pitch adjustable rotor blades; the pitch actuation unit being arranged to: receive an edgewise load signal for each of the adjustable rotor blades; apply an m-blade coordinate transformation to the edgewise load signal to transform the signal to a reference frame along a first and a second reference direction, thereby providing a first signal component and a second signal component; input the first signal component and the second signal component in a primary whirling controller unit to provide a first primary whirling signal component and a second primary whirling signal component, respectively, wherein the primary whirling controller unit comprises: a primary notch filter at a first whirling frequency; a primary signal gain; provide a first resulting signal component as the first primary signal component and a second resulting signal component as the second primary signal component; apply an inverse m-blade coordinate transformation to the first resulting signal component and the second resulting signal component to obtain the pitch modification signals; and apply the pitch modification signals to the pitch actuator.

2. The rotor control system according to claim 1, further comprising: inputting the first signal component and the second signal component in a secondary whirling controller unit to provide a first secondary whirling signal component a second secondary whirling signal component, wherein the secondary whirling controller unit comprises: a secondary notch filter at a second whirling frequency; a secondary signal gain; combine the first secondary whirling component with the first primary signal component to provide the first resulting signal component; and combine the second secondary whirling component with the second primary signal component to provide the second resulting signal component.

3. The rotor control system according to claim 1 further comprising to apply for each of the first signal component and the second signal component a high-pass filter to remove signal frequency content below the rotor frequency of the rotor blades.

4. The rotor control system according to claim 1, wherein at least one of the primary whirling controller unit or the secondary whirling controller unit comprise a phase shift filter.

5. The rotor control system according to claim 1, wherein the first primary whirling signal component and the second primary whirling signal component are rotated with a rotation operator.

6. The rotor control system according to claim 5, wherein at least one of the rotation angle of the first rotation operator or the second rotation operator is based on the collective pitch angle of the rotor blades.

7. The rotor control system according to claim 1, wherein the m-blade coordinate transformation is based on a Coleman transformation.

8. The rotor control system according to claim 1 further comprising: determine a collective pitch reference for the pitch-adjustable rotor blades, the collective pitch reference being determined based on a rotor speed, apply a resulting pitch modification signal to the pitch-adjustable rotor blades, the resulting pitch modification signal being applied to the pitch-adjustable rotor blades individually, and for each individual blade being based on a combined signal of the collective pitch reference and the individual pitch modification signals.

9. The rotor control system according to claim 1 wherein at least one of the primary signal gain or the secondary signal gain comprises a gain scheduling term, the gain scheduling term being dependent upon an operational point of the wind turbine.

10. The rotor control system according to claim 1, wherein at least one of the primary whirling controller unit or the secondary whirling controller unit further comprising an activation element, the activation element being dependent upon an operational point of the wind turbine.

11. The rotor control system according to claim 1, wherein at least one of the primary whirling controller unit or the secondary whirling controller unit further comprises an activation element, the activation element is dependent upon the edgewise load signal of at least one adjustable rotor blade.

12. (canceled)

13. A method of actuating pitch of pitch adjustable rotor blades of a wind turbine, the wind turbine comprises a pitch actuator for actuating the pitch of the pitch adjustable rotor blades, the method comprises: receiving an edgewise load signal for each of the adjustable rotor blades; applying an m-blade coordinate transformation to the edgewise load signal to transform the signal to a reference frame along a first and a second reference direction, thereby providing a first signal component and a second signal component; applying to the first signal component and the second signal component: a primary notch filter at a first whirling frequency; a primary signal gain; thereby providing a first primary whirling signal component and a second primary whirling signal component, respectively; providing a first resulting signal component as the first primary signal component and a second resulting signal component as the second primary signal component; applying an inverse m-blade coordinate transformation to the first resulting signal component and the second resulting signal component to obtain the pitch modification signals; and applying the pitch modification signals to the pitch actuator.

14. (canceled)

15. The rotor control system according to claim 1, wherein the first secondary whirling signal component and the second secondary whirling signal component are rotated with a rotation operator.

16. A wind turbine, comprising: a tower; a nacelle disposed on the tower; a rotor extending from the nacelle and having pitch adjustable rotor blades disposed at a distal end thereof; a pitch actuator for actuating the pitch of the pitch adjustable rotor blades; a rotor control system for actuating pitch of the pitch adjustable rotor blades, the rotor control system comprising a pitch actuation unit for determining pitch modification signals to be applied to the pitch actuator; the pitch actuation unit being arranged to: receive an edgewise load signal for each of the adjustable rotor blades; apply an m-blade coordinate transformation to the edgewise load signal to transform the signal to a reference frame along a first and a second reference direction, thereby providing a first signal component and a second signal component; input the first signal component and the second signal component in a primary whirling controller unit to provide a first primary whirling signal component and a second primary whirling signal component, respectively, wherein the primary whirling controller unit comprises: a primary notch filter at a first whirling frequency; a primary signal gain; provide a first resulting signal component as the first primary signal component and a second resulting signal component as the second primary signal component; apply an inverse m-blade coordinate transformation to the first resulting signal component and the second resulting signal component to obtain the pitch modification signals; and apply the pitch modification signals to the pitch actuator.

17. A computer program product comprising software code adapted to control a wind turbine when executed on a data processing system, the computer program product being adapted to perform an operation of actuating pitch of pitch adjustable rotor blades of the wind turbine, the operation comprising: receive an edgewise load signal for each of the adjustable rotor blades; apply an m-blade coordinate transformation to the edgewise load signal to transform the signal to a reference frame along a first and a second reference direction, thereby providing a first signal component and a second signal component; apply to the first signal component and the second signal component: a primary notch filter at a first whirling frequency; a primary signal gain; thereby providing a first primary whirling signal component and a second primary whirling signal component, respectively; provide a first resulting signal component as the first primary signal component and a second resulting signal component as the second primary signal component; apply an inverse m-blade coordinate transformation to the first resulting signal component and the second resulting signal component to obtain the pitch modification signals; and apply the pitch modification signals to a pitch actuator for actuating the pitch of the pitch adjustable rotor blades.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

[0037] FIG. 1 illustrates, in a schematic view, an example of a wind turbine;

[0038] FIG. 2 is a diagram schematically illustrating an embodiment of a feedback speed controller;

[0039] FIG. 3 schematically illustrates an embodiment of a pitch actuation unit with a Coleman transformation computing block;

[0040] FIG. 4 schematically illustrates an embodiment of an implementation of an activation element;

[0041] FIG. 5 illustrates simulated load signals by applying the embodiment illustrated in FIG. 3; and

[0042] FIG. 6 illustrates simulated pitch signals by applying the embodiment illustrated in FIG. 3.

DESCRIPTION OF EMBODIMENTS

[0043] FIG. 1 illustrates, in a schematic view, an example of a wind turbine 1. The wind turbine 1 includes a tower 2, a nacelle 3 disposed at the apex of the tower, and a rotor 4 operatively coupled to a generator housed inside the nacelle 3. In addition to the generator, the nacelle houses miscellaneous components required for converting wind energy into electrical energy and various components needed to operate, control, and optimize the performance of the wind turbine 1. The rotor 4 of the wind turbine includes a central hub 5 and a plurality of blades 6 that project outwardly from the central hub 5. In the illustrated embodiment, the rotor 4 includes three blades 6, but the number may vary. Moreover, the wind turbine comprises a control system. The control system may be placed inside the nacelle, in the tower or distributed at a number of locations inside (or externally to) the turbine and communicatively connected. The rotor blades are pitch-adjustable. The rotor blades can be adjusted in accordance with a collective pitch setting, where each of the blades are set to the same pitch value. In addition to that, the rotor blades are adjustable in accordance with individual pitch settings, where each blade may be provided with an individual pitch setpoint.

[0044] Each rotor blade of the turbine may vibrate in the edgewise direction 8, that is vibrations or oscillations along the chord between the trailing edge and the leading edge of the blade. In general, when referring to an edgewise rotor blade vibration, such reference is made to the first edgewise bending mode; however, the disclosure made herein is also relevant to higher order edgewise bending mode with the appropriate adaptations. An edgewise vibration may be measured and/or detected in different manners. In one embodiment, the edgewise vibration is measured at the blade root 9 by means of blade load sensors placed at each blade root in a manner so that the sensor detects loading in the edgewise direction. Such sensor may in embodiments be a strain gauge sensor or an optical Bragg-sensor. As the sensors are placed on the rotating blade, such edgewise load signals for each of the adjustable rotor blades are measured in the rotating reference frame of the rotor.

[0045] FIG. 2 is a diagram schematically illustrating an embodiment of a feedback speed controller implemented to determine individual pitch actuation signals capable of reducing edgewise vibrations in accordance with embodiments of the present invention. In the illustrated implementation, the speed controller minimizes a speed error (-.sub.ref) between the actual rotor speed, w, and a reference rotor speed, .sub.ref, in order to output a requested power P (in the form of a power setpoint) and a collective pitch reference, .sub.col. The collective pitch reference as determined by the speed controller, in view of the rotor speed, may also take further sensor values into account, this is referred to in FIG. 2 as a measurement set, ms, being input into the speed controller. The feedback speed controller may be implemented by a PI, PID or similar control schemes. In an embodiment, the speed controller may alternatively be a model predictive controller which based on minimizing a cost function is arranged to determine the collective pitch reference and/or the power reference.

[0046] FIG. 2 further illustrates a vibration reducing control block referred to as pitch actuation units (PAU). In the pitch actuation unit pitch modification signals (.sub.1, .sub.2, .sub.3) are being determined based on input signal(s), the input signal include edgewise load signals. An embodiment of the implementation of the pitch actuation unit (PAU) is illustrated in FIG. 3.

[0047] The PAU control unit determines pitch modification signals (.sub.1, .sub.2, .sub.) for each rotor blade which are superimposed onto the collective pitch reference to provide resulting pitch modification signals (.sub.A, .sub.B, .sub.C) that can be applied to the pitch actuators of the rotor blades individually, and thereby reducing edgewise blade vibrations.

[0048] In the embodiment shown in FIG. 2, a collective pitch reference for the pitch-adjustable rotor blades is being determined based on a rotor speed and a resulting pitch modification signal is being applied to the pitch-adjustable rotor blades. The resulting pitch modification signal being applied to the pitch-adjustable rotor blades individually, and for each individual blade being based on a combined signal of the collective pitch reference and the individual pitch modification signals. In an embodiment, the individual pitch modification signal is being applied in a cyclic manner.

[0049] Thus, pitch actuation signals are determined for each pitch adjustable rotor blade based on the pitch modification signal for each rotor blade.

[0050] FIG. 3 schematically illustrates an embodiment of a pitch actuation unit (PAU) which based on an m-blade coordinate transformation (T), in the form of a Coleman transformation, determines pitch modification signals which when applied by the pitch actuator generates pitch actuation signals that will reduce edgewise blade vibrations.

[0051] The pitch actuation unit PAU is arranged to receive an edgewise load signal for each of the adjustable rotor blades (L.sub.1 to L.sub.3). The edgewise load signal being measured in a rotating reference frame. The edgewise load signals are coordinate transformed by the m-blade coordinate transformation in the form of a Coleman transformation T The Coleman transformation takes the three rotating signals into a fixed reference frame along a first and a second reference direction, thereby providing a first signal component, a, and a second signal component, b.

[0052] In an embodiment, the three edgewise load signals (L.sub.1, L.sub.2, L.sub.3) are transformed from the rotary frame of the rotor to the fixed frame of the nacelle by application of an m-blade coordinate transform in the form of a Coleman transform T using:

[00001]$\left(\begin{array}{c}a\\ b\end{array}\right)=\left(\frac{2}{3}\right)\left(\begin{array}{ccc}\cos \left(\right)& \cos \left(+\frac{2}{3}\right)& \cos \left(+\frac{4}{3}\right)\\ \sin \left(\right)& \sin \left(+\frac{2}{3}\right)& \sin \left(+\frac{4}{3}\right)\end{array}\right)\left(\begin{array}{c}{L}_1\\ L_2\\ L_3\end{array}\right)$

with being the rotor frequency.

[0053] In the illustrated embodiment, a high-pass filter HP is applied to each of the signal components a, b to remove signal frequency content below the rotor frequency of the rotor blades. The high-pass filter is set to remove signal DC frequency content, which may be implemented by removing frequency content substantially below the rotor frequency 1P.

[0054] The first and the second signal components, a and b, are input into a primary whirling controller, C-pri, and optionally, into a secondary whirling controller, C-sec.

[0055] In the illustrated embodiment of FIG. 3, each signal component is input into the primary whiling controller unit as well as the secondary whiling controller unit. In the illustrated embodiment, the primary controller unit C-pri is implemented to target the first backward whirling mode, and is also referred to as a backward whirling controller (BWC). The secondary controller unit C-sec is implemented to target the first forward whirling mode, and is also referred to as a forward whirling controller (FWC).

[0056] The primary whirling controller unit determines a first primary whirling signal component, a.sub.pri-1 and a second primary whirling signal component b.sub.pri-2, respectively. The secondary whirling controller unit determines a first secondary whirling signal component, a.sub.sec-1 and a second secondary whirling signal component b.sub.seci-2, respectively.

[0057] The primary whirling controller (backward whirling controller (BWC)) applies identical actions to both the a and the b signal components. Similarly, the secondary whirling controller (forward whirling controller (FWC)) applies identical actions to the both the a and the b components.

[0058] In the illustrated embodiment, the first signal component, a, is input in the backward whirling controller BWC to determine a first backward whirling component a.sub.pri-1 by applying a notch filter at a forward whirling frequency (first whirling frequency) and a backward whirling signal gain k.sub.pri (primary signal gain) to the first signal component. Additionally, the first signal component, a, is input in the forward whirling controller FWC to determine a first forward whirling component a.sub.sec-1 by applying a notch filter at a backward whirling frequency (second whirling frequency) and a forward whirling signal gain k.sub.sec to (secondary signal gain) the first signal component.

[0059] In a corresponding manner, the second signal component, b, is input in the backward whirling controller BWC to determine a second backward whirling component b.sub.pri-2 by applying a notch filter at a forward whirling frequency and a backward whirling signal gain k.sub.pri to the second signal component. Additionally, the second signal component, b, is input in the forward whirling controller FWC and a second forward whirling component b.sub.sec-2 is determined by applying a notch filter at a backward whirling frequency and a forward whirling signal gain k.sub.sec to the first signal component.

[0060] In a general embodiment, where only the primary whirling controller (C-pri) is used, a first resulting signal component, A, is determined as the first primary signal component and a second resulting signal component, B, is determined as the second primary signal component.

[0061] However, in the illustrated embodiment where also the secondary whirling controller is used, the first secondary whirling component is combined with the first primary signal component to provide the first resulting signal component A. Similarly, the second secondary whirling component is combined with the second primary signal component to provide the second resulting signal component B.

[0062] The resulting signal components are used as inputs into an inverse m-blade coordinate transformation T.sup.1 to obtain the pitch modification signals .sub.1, .sub.2, .sub.3. The pitch modification signals are applied to the pitch actuator.

[0063] The inverse m-blade coordinate transformation may be an inverse Coleman transformation on the general form:

[00002]$\left(\begin{array}{c}{}_1\\ {}_2\\ {}_3\end{array}\right)=\left(\begin{array}{cc}\cos \left(\right)& \sin \left(\right)\\ \cos \left(+\frac{2}{3}\right)& \sin \left(+\frac{2}{3}\right)\\ \cos \left(+\frac{4}{3}\right)& \sin \left(+\frac{4}{3}\right)\end{array}\right)\left(\begin{array}{c}A\\ B\end{array}\right)$

[0064] In the mentioned embodiment filtering comprises applying a notch filter at the backward whirling frequency and the forward whirling frequency. That is a notch filter placed around the edgewise vibration frequency f shifted either backwards or forwards by the rotor frequency, that is a notch filter placed at either (f1P) or (f+1P).

[0065] The whirling controller units includes the application of a gains, referred to as a primary signal gain k.sub.pri and a secondary signal gain k.sub.sec to impose control actions to predefined degrees, as determined by the gains. In an embodiment, the application of the gains is a multiplication of a value to the respective signals.

[0066] There may be a need to adjust the phase of the signal components. In one embodiment the primary whirling controller unit comprises and/or the secondary whirling controller unit further comprises a phase shift filter. The signals may be phase shifted by use of a lead-lag filter in the signal paths, e.g. placed after application of the gains (k.sub.pri, k.sub.sec) thereby imposing a phase shift to adjust the phase of the respective signal component.

[0067] In the embodiment illustrated in FIG. 3, the phase is manipulated by rotating the two components a, b using a rotation operator R which rotates the backward signal components an angle .sub.pri (first rotation operator) and the forward signal components an angle .sub.sec (second rotation operator) By rotation of the signal components, the applied force vector from the resulting pitching can be adjusted along a desired direction. In an embodiment the rotation angle of the first rotation operator and/or the second rotation operator is based on the collective pitch angle of the rotor blades.

[0068] As further shown in the embodiment of FIG. 3, the forward whirling controller unit and the backward whirling controller units comprises a gain scheduling term g.sub.pri, g.sub.sec. The gain scheduling terms applied in the backward whirling controller BWC (g.sub.pri) and the gain scheduling terms applied in the forward whirling controller FWC (g.sub.sec) may be set in the same or different manner. Dependent on the specific situation, there may be a need to treat the forward or backward whirling component differently.

[0069] In an embodiment the gain scheduling terms being dependent upon an operational point of the wind turbine. The pitch angle and/or rotor speed may be used to schedule the gains. For example, a low rotor speed, or at rotor speeds far from the edgewise blade vibration frequency, the gain may be set low. There may also be pitch angle ranges, where it is known that the pitching does not have a large effect on reducing the edgewise vibrations, and therefore the pitch activity may be reduced to reduce the fatigue impact. Likewise, the gain scheduling terms may be set based on a functional relationship with the pitch angle.

[0070] Additionally, further signal treatment may be applied to further improve the signals. For example, further signal treatment may be applied 30 to the edgewise load signals and/or applied 31 to the pitch modifications signals. Such further signal treatment may be further notch filters and band pass filters for removing signal content not relevant for the whirling controller units or the pitch actuator.

[0071] The rotor control system may further comprise an activation element. The activation element may be a dedicated activation element or implemented in the gain scheduling as a zero gain when the control system is deactivated. Other implementations of an activation element are also possible. The activation element may be dependent upon an operational point of the wind turbine. In this manner, it can be ensured that the controller is active in operational conditions such as turbulence or other conditions where there is an elevated risk of edgewise vibrations building up.

[0072] In an embodiment the activation element may be made dependent upon the edgewise load signal of at least one adjustable rotor blade. For example, the signal content of the load signal at the edgewise frequency may be inserted into a root-mean-square RMS filter component, and a threshold may be set to the RMS signal, above which the pitch activation is enabled.

[0073] FIG. 4 illustrates an embodiment where the primary whirling controller unit comprising an activation element of the RMS type being dependent upon the edgewise load signal of at least one adjustable rotor blade. A similar activation element may be implemented also for the secondary whirling controller unit.

[0074] In the illustrated embodiment, the first and second signal components, after passage of the notch filter, are inserted into a gain scheduling unit 40. Each signal component is passed through a bandpass filter (BP) centred at the backward whirling frequency. In general, the bandpass filter is centred at the target frequency, so if the whirling controller unit is a backward whirling controller unit, the bandpass filter is centred at the backward whirling frequency, and vice versa for a forward whirling controller unit. The bandpassed signal is squared (u.sub.1.sup.2; u.sub.2.sup.2), summed and the square root of the sum signal U is evaluated in terms of a lower threshold (l), below which a zero is sent out, and upper threshold (h), above which a unity factor is sent out, and in between where a factor between 0 and 1 is sent out. The output is a scheduling factor g.sub.m, which is multiplied with the relevant signal gain (primary signal gain or secondary signal gain).

[0075] The activation element may include a hysteresis to ensure that the controller is not flipping on and off in certain conditions. The activation element may also include a timer to ensure that the controller is active for a certain time thereby increasing the likelihood that the vibration is broken.

[0076] FIGS. 5 and 6 illustrate simulated signals by applying the embodiment illustrated in FIG. 3.

[0077] FIG. 5A illustrates a time trace of the edgewise load signal of one of the rotor blades for a range between 530 and 600 seconds. While the two signals are somewhat overlaid, one trace 50A is for a situation where the PAU of FIG. 3 is not enabled, and the other trace 51A is for a situation where the PAU of FIG. 3 is enabled. The differences in the two signals are better seen in FIG. 5B which shows an FFT plot of the signals of FIG. 5A. Here, the signal 50B is for the situation where the PAU is not enabled, whereas the signal 51B is for the situation where the PAU is enabled. Signal content is seen at two frequencies, namely at the 1P frequency 52 and the edge frequency 53 of the rotor blade. As can be seen there is not a difference in the signal content at the 1P peak since the rotor frequency is not changed, however a clear reduction of the signal content at the edge frequency is seen, due to the load reduction from the added pitch actuation. Embodiments of the present invention thus provides a rotor control system with the effect of a reduction in edgewise vibrations of the blades of the turbine.

[0078] FIG. 6A illustrates a time trace for the added pitch signal 401 for the same time period. The time trace 60A is a flat line, as the PAU is not enabled, whereas the time trace 61A show the added pitch signal. FIG. 6B shows an FFT plot of the signals of FIG. 6A. As can be seen a significant peak is present at the edge frequency 53 due to the added pitch activity. FIG. 6 shows that the load reduction comes at a cost, namely an increased pitch activity. As a consequence, the skilled person should in connection with the implementation of an embodiment of the invention tune the system to find a balance between load reduction and increased pitch activity.

[0079] Example embodiments of the invention have been described for the purposes of illustration only, and not to limit the scope of the invention as defined in the accompanying claims.