Wind turbine and a method of operating a wind turbine for reducing edgewise vibrations

Abstract

The present invention relates to a wind turbine and a method of operating a wind turbine for reducing edgewise vibrations, wherein the wind turbine comprises a wind turbine control system having at least one vibration sensor for measuring the edgewise vibrations and a controller for receiving the vibration signal from the vibration sensor. The sensor unit is preferably an accelerometer arranged in a stationary frame of reference. The controller determines at least one whirling mode frequency and the vibration level thereof. The controller initiates a corrective action if the measured vibration level exceeds a threshold value. The corrective action is preferably an adjustment of the pitch angle of the wind turbine blades which in turn dampens the edgewise vibrations.

Claims

1. A method of controlling a wind turbine for reducing edgewise vibrations of blades, the method comprising: regulating a wind turbine as a function of a vibration level and frequency of edgewise vibrations, wherein the edgewise vibration level and frequency are determined as a function of a vibration signal of the wind turbine side-to-side vibrations of the tower obtained from the wind turbine, wherein the side-to-side vibrations of the tower are obtained from a single vibration signal obtained in a nacelle of the wind turbine, wherein the frequency of edgewise vibration is determined from the frequency spectrum of the vibration signal by identifying the frequency location of a pair of peaks in the frequency spectrum, wherein the wind turbine is whirling in a rotational direction wherein the vibration signal is analyzed to determine a first whirling mode frequency and a second whirling mode frequency, wherein the first whirling mode frequency is indicative of backward whirling mode movement that occurs in an opposite direction compared to the rotational direction, wherein the second whirling mode frequency is indicative of forward whirling mode movement that occurs in the rotational direction, wherein an edgewise vibration mode frequency is determined from one-half of a sum of the first whirling mode frequency and the second whirling mode frequency, and when the edgewise vibration mode frequency exceeds a predetermined threshold, regulating operation of the wind turbine components.

2. The method according to claim 1, wherein the frequency of edgewise vibration is determined from the frequency spectrum of the vibration signal by identifying at least one peak in a predetermined frequency band.

3. The method according to claim 2, wherein the frequency of edgewise vibration is determined by identifying the frequency location of multiple pairs of peaks or at least one peak in a predetermined frequency band in the frequency spectrum.

4. The method according to claim 2, wherein the level of vibration is determined from the height or power of one or more identified peaks.

5. The method according to claim 1, wherein the act of regulating the operation of the wind turbine comprises one or more acts of: adjusting a pitch angle of the at least one wind turbine blade, adjusting the rotational speed of the rotor of the wind turbine, adjusting a yaw angle of the nacelle of the wind turbine, adjusting a generator torque signal or a power output signal of the wind turbine, and applying a braking force to the at least one wind turbine blade using a braking system of the wind turbine.

6. The method according to claim 5, wherein the one or more acts of adjusting or applying is performed as a transfer function of the vibration level.

7. The method according to claim 6, wherein the pitch angle of the at least one wind turbine blade is pitched if the edgewise vibration mode frequency exceeds the predetermined threshold.

8. A wind turbine having a controller, wherein the controller is configured to execute computer implemented instructions that perform the method as described in claim 1.

9. A wind turbine comprising: a wind turbine tower, a nacelle arranged on top of the wind turbine tower, a rotatable rotor with at least one wind turbine blade arranged relative to the nacelle, and a wind turbine control system, wherein the wind turbine control system comprises a single vibration sensor configured to measure one vibration signal along at least a lateral direction perpendicular to a central rotational axis of the rotor, and a controller connected to receive the vibration signal, wherein the controller is configured to determine at least one frequency of an edgewise vibration based on the vibration signal, wherein the wind turbine is whirling in a rotational direction wherein the vibration signal is analyzed to determine a first whirling mode frequency and a second whirling mode frequency, wherein the first whirling mode frequency is indicative of backward whirling mode movement that occurs in an opposite direction compared to the rotational direction, wherein the second whirling mode frequency is indicative of forward whirling mode movement that occurs in the rotational direction, wherein an edgewise vibration mode frequency is determined from one-half of a sum of the first whirling mode frequency and the second whirling mode frequency, and when the edgewise vibration mode frequency exceeds a predetermined threshold, regulate operation of the wind turbine components.

10. The wind turbine according to claim 9, wherein the wind turbine control system is connected to at least one component selected from a pitch mechanism configured to pitch the at least one wind turbine into a pitch angle, a yaw mechanism configured to yaw the nacelle into a yaw angle, a generator configured to generate a power output signal, a power convertor configured to generate another power output signal, and a braking system configured to apply a braking force to the rotor, wherein the controller is configured to regulate the operation of the wind turbine by adjusting at least one control parameter of the at least one component so that the edgewise vibrations are reduced.

11. The wind turbine according to claim 9, wherein the wind turbine control system is configured to determine the frequency of edgewise vibration determined from a frequency spectrum of the vibration signal by identifying a frequency location pairs of backward and forward peaks in the frequency spectrum.

Description

DESCRIPTION OF THE DRAWING

(1) The invention is described by example only and with reference to the drawings, wherein:

(2) FIG. 1 shows an exemplary embodiment of a wind turbine,

(3) FIG. 2 shows an exemplary graph of a vibration signal measured in a rotating frame of reference,

(4) FIG. 3 shows an exemplary graph of a vibration signal measured in a stationary frame of reference,

(5) FIG. 4 shows an exemplary graph of two transfer functions as function of the vibration level, and

(6) FIG. 5 shows an exemplary flowchart of the control method according to the invention.

(7) In the following text, the figures will be described one by one, and the different parts and positions seen in the figures will be numbered with the same numbers in the different figures. Not all parts and positions indicated in a specific figure will necessarily be discussed together with that figure.

REFERENCE LIST

(8) 1. Wind turbine 2. Wind turbine tower 3. Nacelle 4. Yaw mechanism 5. Wind turbine blades 6. Rotor hub 7. Pitch mechanism 8. Generator 9. Tip end 10. Blade root 11. Leading edge 12. Trailing edge 13. Braking system 14. Vibration sensor 15. Vibration signal measured in the wind turbine blade 16. First whirling mode 17. Second whirling mode 18. Vibration signal measured in the wind turbine tower 19. First transfer function 20. Second transfer function 21. Vibration level 22. Pitch angle offset

DETAILED DESCRIPTION OF THE INVENTION

(9) FIG. 1 shows a wind turbine 1 comprising a wind turbine tower 2 and a nacelle 3 arranged on top of the wind turbine tower 2 using a yaw mechanism 4. The yaw mechanism 4 is configured to yaw the nacelle 3 into a yaw angle. A rotor comprising at least two wind turbine blades 5 mounted to a rotor hub 6 via a pitch mechanism 7.

(10) The pitch mechanism 7 is configured to pitch the wind turbine blades 5 into a pitch angle. The rotor hub 6 is rotatably connected to a generator 8 arranged in the wind turbine 1 via a rotor shaft.

(11) Each wind turbine blade 5 comprises a tip end 9 and a blade root 10, wherein the wind turbine blade 5 has an aerodynamic profile defining a leading edge 11 and a trailing edge 12. A braking system 13 is arranged relative to the rotor and is configured to apply a braking force to the rotor. At least one vibration sensor 14 is arranged on the wind turbine 1 for measuring a vibration signal of the wind turbine 1. The vibration sensor 14 forms part of a wind turbine control system wherein a controller is connected to the vibration sensor 14.

(12) FIG. 2 shows an exemplary graph of a vibration signal 15 measured in a rotating frame of reference, wherein the vibration signal 15 is measured in the wind turbine blade 5. The vibration signal 15 is here shown as an acceleration signal indicative of the edgewise vibrations in the wind turbine blades 5.

(13) The rotational speed of the rotor may be determined by analysing the vibration signal 15, wherein the peak marked “1P” indicates the rotational frequency of the rotor. The vibration signal 15 further includes a plurality of individual edgewise modes indicative of the edgewise vibrations. Here, only a first edgewise mode 16 and a second edgewise mode 17 are shown. The vibration signal 15 may be analysed to determine a blade edgewise frequency of the first edgewise mode 16 and a blade edgewise frequency of the second edgewise mode 17. The individual edgewise modes 16, 17 are monitored and analysed individually and in parallel.

(14) FIG. 3 shows an exemplary graph of another vibration signal 18 measured in a stationary frame of reference, wherein the edgewise vibrations in the wind turbine blades 5 are indirectly determined by measuring a vibration signal using a vibration sensor located in the wind turbine tower 2 or the nacelle 3. The vibration signal 18 is measured in a lateral direction perpendicular to the rotation axis of the rotor.

(15) The vibration signal 18 is analysed to determine a first whirling mode frequency, a, and a second whirling mode frequency, b, of each of the edgewise modes 16, 17. The first whirling mode frequency is indicative of a backward whirling mode movement that occurs in an opposite direction compared to the rotational direction. The second whirling mode frequency is indicative of a forward whirling mode movement that occurs in the same direction compared to the rotational direction. The rotational frequency, 1P, is used to determine a whirling mode range for each whirling mode 16, 17.

(16) The frequency spectrum of the vibration signal 18 obtained from vibration measurements of the tower 2 or the nacelle 3 shows a first pair of peaks a.sup.1 and b.sup.1 corresponding to a first edgewise vibration mode 16 seen in FIG. 2. The peak a1 is located at f.sub.edge-1P, where 1P is the rotation frequency, and is the “backward whirling mode”. The peak b.sup.1 is located at f.sub.edge+1P and is the “forward whirling mode”.

(17) The frequency of the first edgewise vibration mode 16 may then be determined as f.sub.edge=(f.sub.peak a+f.sub.peak b)/2.

(18) Likewise, the second pair of peaks a.sup.2 and b.sup.2 corresponds to a second edgewise vibration mode 17 seen in FIG. 2.

(19) Identifying one or both of peaks a and b in the frequency spectrum from a vibration simply obtained from vibration measurements in the tower or in the nacelle will allow for determining the corresponding edgewise vibration mode.

(20) One peak, a or b, may also be determined by looking in a band or frequency range in the spectrum. The band or frequency may be 1P, 2P or in that order. A peak identified in such a band may indicate the frequency of a corresponding edgewise mode.

(21) The vibration signal 18 is transformed into the frequency domain using a frequency transformation function. The frequency transformed signal is analysed and at least the vibration levels of the first and second whirling mode frequencies are determined.

(22) As indicated in FIGS. 2 and 3, the vibration signal 15, 18 includes a number of critical frequencies, e.g. the eigenfrequencies of the wind turbine tower marked “Tower”, of the drivetrain of the wind turbine 1 marked “DT”, and of the passing frequency of the wind turbine blades marked “3P”. The individual critical frequencies may be taken into account when determining the vibration level of the respective edgewise modes 16, 17.

(23) FIG. 4 shows an exemplary graph of two transfer functions 19, 20 used to determine a corrective action. A first transfer function 19 is applied to the vibration level 21 of the first edgewise mode 16 and a second transfer function 20 is applied to the vibration level 21 of the second edgewise mode 17.

(24) In this embodiment, the transfer functions 19, 20 are used to determine a pitch angle offset 22 for adjustment of the pitch angle of the wind turbine blades 5. The pitch angle offset 22 is optionally compared to a predetermined threshold value. If the pitch angle offset 22 exceeds this threshold value, then the controller initiates a shutdown procedure and the wind turbine 1 is shut down until the vibration level 21 drops below the respective threshold value. The pitch angle offset is used by the pitch mechanism to pitch the wind turbine blades 5 out of the wind relative to a normal pitch angle. The pitching of the wind turbine blades 5 has significant dampening effect of the whirling mode vibrations in the wind turbine blades 5.

(25) FIG. 5 shows an exemplary flowchart of the control method according to the invention. The controller of the wind turbine control system monitors the whirling modes within the operating range of the rotational speed of the rotor. The vibration signal 15, 18 is measured using the vibration sensor 14 and transferred to the controller.

(26) The controller analyses the vibration signal, e.g. in the frequency domain, to determine the first and second whirling mode frequencies of the individual whirling modes 16, 17. The controller further determines the individual vibration levels 21 using the whirling mode frequencies.

(27) The respective vibration levels 21 are then compared to individual threshold values in the controller. If the vibration levels 21 are below the threshold values, then the controller continues to monitor the whirling mode vibrations. If at least one of the vibration levels 21 exceeds its respective threshold value, then the controller determines a suitable corrective action, e.g. a pitch angle offset. Suitable control parameters are then transmitted to the respective component, e.g. the pitch mechanism, which in turn adjusts its operation accordingly.