Position based vibration reduction of nacelle movement of wind turbine

Abstract

The present invention relates to control of a wind turbine where nacelle vibration is reduced by use of blade pitching or generator torque modulation. The nacelle vibrations are reduced based on a position signal of the nacelle. An actuator signal is determined based on the position signal and applied to the actuator capable of reducing nacelle vibration. The actuator signal is gain adjusted based on a separation between the rotor frequency and tower vibration frequency.

Claims

1. A method of controlling a wind turbine comprising a tower structure supporting a nacelle and a rotor with a number of pitch-adjustable rotor blades, the rotor being arranged to drive an electrical generator, the method comprising: obtaining a position signal indicative of a position of the nacelle in a direction of a movement of the nacelle; determining, based on the position signal, a first signal configured to reduce nacelle vibration in the direction of the movement of the nacelle; determining a tower vibration frequency of a tower vibration eigenmode; determining an excitation frequency affecting the tower structure; determining an adjustment gain having an operational sign based on a difference, or on a ratio, between the excitation frequency and the tower vibration frequency; applying the adjustment gain to the first signal; determining an actuator signal based on the gain-adjusted first signal; and applying the actuator signal to an actuator of the wind turbine capable of reducing the nacelle vibration.

2. The method according to claim 1, further comprising: obtaining a velocity signal indicative of a velocity of a movement of the nacelle in the direction of the movement of the nacelle; determining, based on the velocity signal, a second signal configured to reduce the nacelle vibration; and applying the adjustment gain to the second signal, wherein determining the actuator signal is further based on the pain-adjusted second signal.

3. The method according to claim 2, further comprising: obtaining, using an accelerometer of the wind turbine, an acceleration signal in the direction of the movement of the nacelle, wherein obtaining the velocity signal comprises: applying an integration of the acceleration signal to obtain an estimated velocity signal indicative of a velocity of a movement of a tower top.

4. The method according to claim 3, wherein the second signal is determined as the estimated velocity signal multiplied with a second gain.

5. The method according to claim 1, further comprising: determining, based on a rotor speed, a collective pitch reference for the pitch-adjustable rotor blades; and applying, to each of the pitch-adjustable rotor blades individually, a resulting pitch signal that is based on a combined signal of the collective pitch reference and the actuator signal.

6. The method according to claim 1, further comprising: transforming the actuator signal from a signal representing a desired force or torque in the direction of the movement of the nacelle to a resulting pitch contribution for each of the pitch-adjustable rotor blades.

7. The method according to claim 1, further comprising: determining, based on a rotor speed, a power reference for the electrical generator; and applying, to the electrical generator, a resulting power reference signal that is based on a combined signal of the power reference and the actuator signal.

8. The method according to claim 1, further comprising: determining, using feedback control based on minimizing a speed error between an actual rotor speed and a reference rotor speed, one of a collective pitch reference for the pitch-adjustable rotor blades or a power reference for the electrical generator.

9. The method according to claim 1, further comprising: obtaining, using an accelerometer of the wind turbine, an acceleration signal in the direction of the movement of the nacelle, wherein obtaining the position signal comprises: applying a first integration of the acceleration signal to obtain an estimated velocity signal; and applying a second integration of the estimated velocity signal to obtain an estimated position signal indicative of a position of a tower top in the direction of the movement of the nacelle.

10. The method according to claim 9, wherein the first signal is determined as the estimated position signal multiplied with a first gain.

11. The method according to claim 1, wherein the position signal is high-pass filtered prior to determining the first signal.

12. The method according to claim 1, wherein the excitation frequency is a selected positive integer times a rotor frequency.

13. The method according to claim 1, wherein the direction of the movement of the nacelle is a lateral direction or a torsional direction.

14. The method according to claim 1, wherein determining the adjustment gain is further based on a function having a functional dependency upon a rotor speed.

15. The method according to claim 1, wherein the adjustment gain further comprises a gain scheduling term that is dependent upon an operational point of the wind turbine.

16. The method according to claim 1, wherein the actuator of the wind turbine is capable of reducing the nacelle vibration in a lateral direction of the movement of the nacelle, wherein the actuator signal is one or both of: a power reference adjustment configured to vary, in dependence on a lateral vibration, a counter torque of the electrical generator over time to provide a generator torque-induced force that reduces the lateral vibration, and a blade pitch angle adjustment for each of the pitch-adjustable rotor blades to provide a lateral force that counteracts the lateral vibration.

17. A computer program product stored on a non-transitory medium and comprising software code adapted to perform an operation for controlling a wind turbine when executed on a data processing system, the wind turbine comprising a tower structure supporting a nacelle and a rotor with a number of pitch-adjustable rotor blades, the rotor being arranged to drive an electrical generator, the operation comprising: obtaining a position signal indicative of a position of the nacelle in a direction of a movement of the nacelle; determining, based on the position signal, a first signal configured to reduce nacelle vibration in the direction of the movement of the nacelle; determining a tower vibration frequency of a tower vibration eigenmode; determining an excitation frequency affecting the tower structure; determining an adjustment gain having an operational sign based on a difference, or on a ratio, between the excitation frequency and the tower vibration frequency; applying the adjustment gain to the first signal; determining an actuator signal based on the gain-adjusted first signal; and applying the actuator signal to an actuator of the wind turbine capable of reducing the nacelle vibration.

18. A control system for a wind turbine comprising a tower structure supporting a nacelle and a rotor with a number of pitch-adjustable rotor blades, the rotor being arranged to drive an electrical generator, the control system comprising: an input module configured to obtain a position signal indicative of a position of the nacelle in a direction of a movement of the nacelle; a processing module configured to: determine, based on the position signal, a first signal configured to reduce nacelle vibration in the direction of the movement of the nacelle; determine a tower vibration frequency of a tower vibration eigenmode; determine an excitation frequency affecting the tower structure; determine an adjustment gain having an operational sign based on a difference, or on a ratio, between the excitation frequency and the tower vibration frequency; apply the adjustment gain to the first signal; and determine an actuator signal based on the gain-adjusted first signal; and an actuator system configured to apply the actuator signal to an actuator of the wind turbine capable of reducing the nacelle vibration.

19. The control system of claim 18, wherein the direction of the movement of the nacelle is a lateral direction or a torsional direction.

20. A wind turbine comprising: a tower structure supporting a nacelle; a rotor with a number of pitch-adjustable rotor blades, the rotor operatively coupled to an electrical generator housed inside the nacelle; and a control system configured to: obtain a position signal indicative of a position of the nacelle in a direction of a movement of the nacelle; determine, based on the position signal, a first signal configured to reduce nacelle vibration in the direction of the movement of the nacelle; determine a tower vibration frequency of a tower vibration eigenmode; determine an excitation frequency affecting the tower structure; determine an adjustment gain having an operational sign based on a difference, or on a ratio, between the excitation frequency and tower vibration frequency; apply the adjustment gain to the first signal; and determine an actuator signal based on the gain-adjusted first signal; and apply the actuator signal to an actuator of the wind turbine capable of reducing the nacelle vibration.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

(2) FIG. 1 illustrates a wind turbine (FIG. 1A) together with a schematic view of lateral vibrational movement (FIG. 1B) and a torsional vibrational movement (FIG. 1C);

(3) FIG. 2 illustrates a general control scheme for an embodiment;

(4) FIG. 3 illustrates a general embodiment of a lateral tower vibration reduction block;

(5) FIG. 4 illustrates an example of the adjustment gain in dependency upon the rotor frequency; and

(6) FIG. 5 and FIG. 6 illustrate example plots.

DESCRIPTION OF EMBODIMENTS

(7) FIG. 1A illustrates, in a schematic perspective 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 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 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 setting.

(8) The turbine may vibrate in the lateral direction 7A, that is in a direction of the rotor plane. Such vibration is also sometimes referred to as side-side vibration. Aspects of lateral vibration is schematically illustrated in FIG. 1B. In this figure, the turbine 10 is schematically illustrated by a tower structure fixed in one end and provided with a mass at the free end. When the tower top vibrates in the lateral direction 7A, the position, x, characteristic of the nacelle position, varies between two maxima defined by the maximum tower deflection during the vibration. The position, x, may be a position representative of the nacelle position in a direction defined by the lateral movement. The position may, e.g., be a centre-of-mass position of the nacelle, the position of the relevant sensor, or position of other fix-points representing the movement of the nacelle in the lateral direction. In addition to the lateral vibration, also torsional vibration along the torsional direction 7B is illustrated in FIG. 1C. In this regard torsional direction should be understood as movement along a path defined by an arc sector or circle sector as shown. When the tower top vibrates in the torsional direction 7B, the position, x′, characteristic of the nacelle position in the torsional direction, varies between two maxima defined by the maximum tower twist during the vibration. In an embodiment using an accelerometer 8 or gyroscope 8 to determine the nacelle movement, the accelerometer/gyroscope should be properly placed, such as at one end of the nacelle. In this regard, it may not be necessary to directly detecting the movement along the torsional direction, instead also indirect detection of the movement may be used, as long as the indirect movement correlates with the torsional movement. This may e.g. be obtained by an accelerometer which detects movement in the lateral direction, in combination with further information to determine that the movement is related to torsional vibration. Such further information may be the frequency of the vibration.

(9) In a general embodiment of the present invention vibrational movement is reduced by the following general steps: Obtain a position signal indicative of a position of the nacelle in the relevant movement, i.e. determine x (or x′). Based on the position signal determining a signal for damping the nacelle movement, and finally applying the signal to an actuator capable of reducing the vibration.

(10) FIG. 2 is a diagram schematically illustrating this general embodiment in the context of a feedback speed controller. In one implementation, the speed controller minimizes a speed error (ω-ω.sub.ref) between the actual generator speed, ω, and a reference speed, ω.sub.ref, in order to output a requested power P and a collective pitch reference, θ.sub.col. The feedback speed controller may be implemented by a PI, PID or similar control schemes.

(11) FIG. 2 further illustrates a number of vibration reducing control blocks.

(12) A block is illustrated which shows the lateral tower vibration reduction by using the pitch (LTVR-pitch), where pitch actuation signals for reducing lateral tower vibrations is being determined based on the first signal and optionally the second signal. Embodiments of the first and second signals are illustrated in FIG. 3. The LTVR-pitch control block determines an actuator signal θ.sub.P which is transformed in a transformation unit T.sub.P to pitch reference offsets (θ.sub.1, θ.sub.2, θ.sub.3) for each of the blades so that resulting pitch signals (θ.sub.A, θ.sub.B, θ.sub.C) can be applied to the pitch-adjustable rotor blades individually. Each individual blade actuation signal being based on the actuator signal θ.sub.P, and thereby on a combined signal of the collective pitch reference θ.sub.col and the first signal, or a combined signal of the collective pitch reference and the first signal and the second signal as determined by the LTVR-pitch block.

(13) The LTVR-pitch block determines a signal representing a desired force or torque in the direction of the movement of the nacelle. The transformation is to obtain resulting pitch contributions (θ.sub.1, θ.sub.2, θ.sub.3) for each of the pitch-adjustable rotor blades.

(14) The transformation, T.sub.P, may be based on a multi-blade coordinate transformation of the Coleman transformation or Fourier coordinate transformation type, which is arranged to take a signal in a non-rotating reference frame (θ.sub.P) and transform it to a resulting signal in the rotating frame (θ.sub.1, θ.sub.2, θ.sub.3).

(15) As an addition or as an alternative, the lateral tower vibration may also be reduced by using the power as actuator (LTVR-power), where a power actuation signal for reducing lateral tower vibrations by use of the power reference is being determined based on the first signal and optionally the second signal. The LTVR-power control block determine a power reference offset (P.sub.offset) to be combined with the power reference P to provide a resulting power reference (P.sub.set). The resulting power reference is being determined based on a combined signal of the power reference and the actuator signal P.sub.offset, and thereby on the first signal, or a combined signal of the power reference and the first signal and the second signal. The resulting power reference signal is applied to the electrical generator. Embodiments of the first and second signals are illustrated in FIG. 3.

(16) As an addition or as an alternative, also the torsional tower vibration may also be reduced by using the pitch as actuator (LTVR-torsion), where pitch actuation signals for reducing torsion tower vibrations is being determined based on the first signal and optionally the second signal. The LTVR-torsion control block determines an actuator signal θ.sub.T which is transformed in a transformation unit T.sub.P to pitch reference offsets (θ.sub.T1, θ.sub.T2, θ.sub.T3) for each of the blades so that resulting pitch signals (θ.sub.A, θ.sub.B, θ.sub.C) can be applied to the pitch-adjustable rotor blades individually. Each individual blade actuation signal being based on the actuator signal θ.sub.P, and thereby on a combined signal of the collective pitch reference θ.sub.col and the first signal, or a combined signal of the collective pitch reference and the first signal and the second signal as determined by the LTVR-torsion block. Embodiments of the first and second signals are illustrated in FIG. 3.

(17) The LTVR-torsion block corresponds to the LTVR block, and the transformation, T.sub.T, is similar to the transformation T.sub.P, expect that the specific implementation is made for torsional movement.

(18) Moreover, vibration reduction in the fore-aft direction may also be target by imposing a vibration reduction pitch offset signal onto the collective pitch reference θ.sub.col. This pitch offset signal may be determined in a fore-aft vibration reduction block (FAVR), to provide a reduction of the vibration, or damping of the nacelle movement, in the fore-aft direction.

(19) The collective pitch reference is determined by the speed controller in view of the rotor speed and possibly also further sensor values, referred to in FIG. 2 as a measurement set, ms.

(20) FIG. 3 illustrates a general embodiment of a lateral tower vibration reduction block. In this regard it is understood that the tower vibration reduction blocks relating to power, pitch and torsion as illustrated in FIG. 2 could all be implemented according to this general scheme. However, the specific implementation would differ for the different actuation types. In particular the various filters and gains would be adapted to the specific actuation block or actuation type.

(21) FIG. 3 illustrates an embodiment of the LTVR blocks of FIG. 2, where a measured accelerometer signal, a, is used as input, and obtained by an accelerometer positioned so that the acceleration of the movement in the relevant direction of the nacelle is measured, cf. ref. 8 on FIG. 1.

(22) The acceleration signal may in general be used as a raw signal, however typically the signal is pre-processed PP to a certain extend. Such pre-processing may be the application of an anti-aliasing filter to remove any high frequency content which is not needed for the further use. Other filters, including other band-pass filter may be applied during the pre-processing.

(23) The acceleration signal (or pre-processed version of it) is further processed by applying a series of filters to the signal. In the illustrated embodiment, an estimated position signal, x (or x′), indicative of a position of the tower top in the relevant direction is obtained by applying in series a first integration (F1) of the acceleration signal to obtain an estimated velocity signal, v (or v′), and a second integration (F2) of the velocity signal to obtain the position signal, x (or x′). In general any suitable filters which integrate the input signal can be applied. In an embodiment, the first and second integrations may be implemented as leaky integrators. The leaky integrators can be implemented as 1st order low pass filters tuned with a break frequency below the 1st fore-aft mode frequency, the frequency being the system frequency comprising the tower, rotor, nacelle, and optionally also foundation.

(24) The first signal to the actuator capable of reducing the nacelle vibration in the relevant direction (pitch or power) may be determined as the estimated position, x, multiplied with a first gain G1.

(25) In an embodiment, the speed signal indicative of a speed of a movement of the tower top in the relevant direction may be obtained as the estimated velocity signal v, which results after the first integration F1.

(26) The second signal may be determined as the estimated velocity, v, multiplied with a second gain G2.

(27) In this embodiment, the resulting signal is sum of the first (position) and second (velocity) signals. As described, the invention may in an embodiment be implemented by basing the first signal only. In such an embodiment, this may be obtained by setting the gain G2 to zero.

(28) In a further embodiment, also illustrated in FIG. 3, the position signal is high-pass filtered (HP) prior to determining the first signal.

(29) A general advantage of the embodiment described in connection with FIGS. 2 and 3 is that position and velocity measurements or estimates do not have to have correct absolute values, as long as the signals correlate with the real values in the frequency area of interest.

(30) FIG. 4 illustrates an example of the adjustment gain in dependency upon the rotor frequency. That is for a situation where the excitation frequency is the rotor frequency. Thus for a situation where the excitation is imposed by the rotating rotor.

(31) The adjustment gain is applied to the first signal and optionally the second signal, in order to gain adjust the first signal, and optionally the second signal, prior to applying the signals to the actuator of the wind turbine capable of reducing nacelle vibration in the direction of the movement of the nacelle.

(32) In this regard, the tower vibration frequency of a tower vibration eigenmode is determined and a rotor frequency corresponding to the rotor speed is determined. Based on these values, the adjustment gain is determined so that the operational sign of it is defined by a separation between the rotor frequency and tower vibration frequency.

(33) In this regard, the actuation signal S.sub.act may be expressed as:
S.sub.act=g.sub.adj(g.sub.1.Math.x.sub.est+g.sub.2.Math.V.sub.est)

(34) In the embodiment illustrated in FIG. 4 the adjustment gain is set to be 0 for rotor frequencies outside an adjustment zone and increasing positive for rotor frequencies approaching the 1.sup.st eigenmode from lower frequencies, and increasing negative for rotor frequencies approaching the 1.sup.st eigenmode from higher frequencies.

(35) In FIG. 4, the adjustment gain function is illustrated as a piecewise linear function, however, this function may be defined in accordance with any function with a functional dependency upon the rotor speed which express that the operational sign of the adjustment gain is determined by a separation between the excitation frequency and tower vibration frequency.

(36) In embodiments, the separation between the excitation frequency and tower vibration frequency is based on a difference between the excitation frequency and tower vibration frequency or on a ratio between the excitation frequency and tower vibration frequency.

(37) In an embodiment, the actuation signal S.sub.act may further be gain scheduled by including into the adjustment gain a gain scheduling term being dependent upon an operational point of the wind turbine. For example, the gain adjustment term may be multiplied by a factor which increases with increasing acceleration in the lateral direction.

(38) FIG. 5 illustrates example plots in the form of time traces obtained by the method as defined in FIG. 3 compared to same situation without application of the method. The plots are based on a simulation of a wind turbine system.

(39) FIG. 5A illustrates an example pitch settings for a reference situation 51 compared to the corresponding pitch settings 50 for a situation where lateral vibrations are reduced based on embodiments of the present invention. The plot illustrates the blade pitch angle in degrees, the plot referred to as 50 corresponds e.g. to θ.sub.1.

(40) FIG. 5B illustrates the resulting tower top lateral position for the reference situation 53 and the situation where lateral vibration reduction is applied 51. As can be seen directly, the tower top vibration amplitude has been reduced.

(41) The simulations are done for a situation where the rotor frequency is coinciding with the first eigenmode of the lateral vibration. FIG. 5C illustrates a fast Fourier transformation (FFT) of the two signals of FIG. 5B. Thus the figure shows the FFT plots for the reference situation 54 and for the situation based on embodiments of the present invention 55.

(42) As can be seen, the tower frequency is pushed away from the tower vibration frequency, thereby increasing the distance between the rotor frequency and the eigenfrequency.

(43) FIG. 6 illustrates example plots in the form of time traces obtained by the method as defined in FIG. 3 compared to same situation without application of the method. The plots are based on a simulation of a wind turbine system.

(44) FIG. 6A illustrates an example pitch settings for a reference situation 61 compared to the corresponding pitch settings 60 for a situation where torsional vibrations are reduced based on embodiments of the present invention. The plot illustrates the blade pitch angle in degrees, the plot referred to as 60 corresponds e.g. to θ.sub.T1.

(45) FIG. 6B illustrates the resulting nacelle yaw speed which is a measure for the torsional vibration for the reference situation 63 and the situation where torsional vibration reduction is applied 62. As can be seen directly, the tower top vibration amplitude has been reduced.

(46) FIG. 6C illustrates a fast Fourier transformation (FFT) of the two signals of FIG. 6B. Thus the figure shows the FFT plots for the reference situation 64 and for the situation based on embodiments of the present invention 65.

(47) As can be seen, the peak at the observed frequency has been reduced which show that a reduction in the torsional vibrational has been achieved.

(48) Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The invention can be implemented by any suitable means; and the scope of the present invention is to be interpreted in the light of the accompanying claim set. Any reference signs in the claims should not be construed as limiting the scope.