Counteracting tower oscillations of an idling wind turbine

Abstract

The invention presents a method for operating a horizontal axis wind turbine, the wind turbine comprising a tower and a rotor with at least one rotor blade, the rotor being connected to the tower, and being adapted to drive a generator connected to a utility grid, wherein a pitch angle of each rotor blade is adjustable, the method comprising detecting, when the wind turbine is in an idling power producing situation in relation to the utility grid, a tower oscillation, and controlling, when the wind turbine is in the idling power producing situation, the pitch angle of the at least one rotor blade so as to produce aerodynamic forces counteracting the detected tower oscillation.

Claims

1. A method for operating a horizontal axis wind turbine, the wind turbine comprising a tower and a rotor with at least one rotor blade, the rotor being connected to the tower and being adapted to drive a generator connected to a utility grid, wherein a pitch angle of each rotor blade is adjustable, the method comprising: detecting, when the wind turbine is in an idling power producing situation in relation to the utility grid, a tower oscillation; and individually and cyclically controlling, when the wind turbine is in the idling power producing situation, the pitch angle of the at least one rotor blade according to a sinusoidal function so as to produce active damping with aerodynamic forces counteracting the detected tower oscillation, wherein the sinusoidal function is based on time, on an angular position of the at least one rotor blade in a plane of the rotor, and on a rotation frequency of the rotor.

2. The method according to claim 1, further comprising detecting that the wind turbine is in an idling power producing situation in relation to the utility grid.

3. The method according to claim 1, further comprising controlling the wind turbine so as to be in an idling power producing situation in relation to the utility grid.

4. The method according to claim 3, wherein controlling the wind turbine so as to be in an idling power producing situation comprises keeping each blade in a substantially feathered position.

5. The method according to claim 3, wherein controlling the wind turbine so as to be in the idling power producing situation comprises controlling the rotation speed of the rotor by adjusting the pitch angle of the at least one rotor blade.

6. The method according to claim 5, wherein the rotation speed of the rotor is controlled to be within a range from 10 to 25 percent of the nominal rotation speed of the rotor.

7. The method according to claim 1, wherein the wind turbine being in an idling power producing situation in relation to the utility grid includes keeping each blade in a substantially feathered position.

8. The method according to claim 7, wherein keeping each blade in a substantially feathered position includes keeping each blade feathered out into the wind and the pitch angle of each blade at least 70 degrees with reference to a zero degree reference blade position in which a reference chord of the blade is parallel to the rotor plane.

9. The method according to claim 7, wherein keeping each blade in a substantially feathered position includes keeping each blade feathered out into the wind and the pitch angle of each blade at least 80 degrees with reference to a zero degree reference blade position in which a reference chord of the blade is parallel to the rotor plane.

10. The method according to claim 7, wherein keeping each blade in a substantially feathered position includes keeping each blade feathered out into the wind and the pitch angle of each blade at least 86 degrees with reference to a zero degree reference blade position in which a reference chord of the blade is parallel to the rotor plane.

11. The method according to claim 1, wherein detecting the tower oscillation comprises detecting an acceleration of the tower.

12. The method according to claim 1, wherein detecting the tower oscillation comprises detecting a longitudinal tower oscillation, and wherein controlling the pitch angle of the at least one rotor blade comprises controlling the pitch angle of the at least one rotor blade so as to produce aerodynamic forces counteracting the detected longitudinal tower oscillation.

13. The method according to claim 1, wherein detecting the tower oscillation comprises detecting a lateral tower oscillation, and wherein controlling the pitch angle of the at least one rotor blade comprises controlling the pitch angle of the at least one rotor blade so as to produce aerodynamic forces counteracting the detected lateral tower oscillation.

14. The method according to claim 1, wherein the pitch angle of each rotor blade is adjusted by a pitch adjustment system based on a collective pitch reference and input from an acceleration sensor, wherein power to the pitch adjustment system for controlling the pitch angle is received from the utility grid.

15. The method according to claim 1, wherein the pitch angle of each rotor blade is adjusted by a pitch adjustment system based on a collective pitch reference and input from an acceleration sensor, wherein power to the pitch adjustment system for controlling the pitch angle is received from an auxiliary power source.

16. A wind turbine comprising: a tower comprising a rotor having a plurality of rotor blades disposed around a hub at unique angular positions, the rotor connected to the tower and adapted to drive a generator connected to a utility grid, wherein a pitch angle of each rotor blade of the plurality of rotor blades is adjustable; and a controller configured to detect the wind turbine in an idling power producing situation in relation to the utility grid, the controller further configured to detect a tower oscillation, wherein the controller is configured to actively counteract the detected tower oscillation by cyclically adjusting the pitch angle of at least one rotor blade of the plurality of rotor blades according to a sinusoidal function based on time, on an angular position of the at least one rotor blade in a plane of the rotor, and on a rotation frequency of the rotor in response to the detected tower oscillation while in the idling power producing situation.

17. The wind turbine according to claim 16, wherein the detection of the tower oscillation comprises detecting a longitudinal tower oscillation.

18. The wind turbine according to claim 17, wherein the detection of the tower oscillation further comprises detecting a lateral tower oscillation, and wherein adjusting the pitch angle further comprises adjusting at least one of the plurality of rotor blades in response to the lateral tower oscillation.

19. A wind turbine tower controller comprising: a speed control unit that generates a static idling pitch reference based on a rotor speed reference and a measured rotor speed, wherein the speed control unit is configured as a feedback controller that determines the static idling pitch reference, based on a difference between a received rotor speed reference and a measured rotor speed; a longitudinal damper unit that produces a collective pitch reference offset based on an input from a tower longitudinal acceleration sensor; and a lateral damper unit that produces a plurality of individual cyclic pitch reference offsets based on an input from a tower lateral acceleration sensor; wherein both the static idling pitch reference and the collective pitch reference offset are added to at least one of the plurality of individual cyclic pitch reference offsets based on a sinusoidal function of time, of an angular position of at least one rotor blade in a plane of the rotor, and of a rotation frequency of the rotor to generate an active pitch angle dampening control applied to the at least one rotor blade of the wind turbine while the wind turbine is determined to be in an idling power producing situation in relation to a utility grid.

20. The wind turbine tower controller according to claim 19, wherein the speed controller is adapted to provide a generator torque reference for a generator, and wherein the determination of the wind turbine to be in the idling power producing situation comprises determining that the generator torque reference is zero, and the static idling pitch reference is constant.

Description

DESCRIPTION OF FIGURES

(1) Below embodiments of the invention will be described with reference to the drawings, in which

(2) FIG. 1 shows a perspective view of an offshore wind turbine,

(3) FIG. 2 shows a block diagram depicting a control algorithm for carrying out a method according to a preferred embodiment of the invention,

(4) FIG. 3 shows a speed controller of the control algorithm configured as a feedback controller,

(5) FIG. 4 illustrates control of rotation speed of the rotor dependent on wind speed and generated power dependent on the wind speed, and

(6) FIG. 5 illustrates tower oscillations during an idling power producing situation which are damped by an oscillating damping force.

DETAILED DESCRIPTION

(7) FIG. 1 shows an offshore horizontal axis wind turbine 1. The wind turbine comprises a tower 2 supported by an offshore foundation 3. A nacelle 4 is mounted on top of the tower 2, and can rotate in relation to the tower, by means of a yaw system, around a vertical axis. A rotor 5 is mounted on the nacelle 4, and comprises three rotor blades 51. The rotor 5 is connected to a generator 6 in the nacelle 4, via a gearbox 7, and is adapted to drive the generator 6, which in turn is connected to a utility grid 8.

(8) A pitch angle of each rotor blade is adjustable by means of a pitch adjustment system as indicated by the arrows A. A controller 9 is adapted to control the pitch adjustment system based on input from an acceleration sensor 10 in the nacelle as described closer below.

(9) As can be seen in FIG. 2, the controller 9 comprises a speed control unit 901 for controlling the rotational speed of the rotor 5, based on input data 902 representing wind speed etc. The speed controller 901 is adapted to provide a generator torque reference 903 to a generator controller (not shown), and a collective pitch reference 904 to the pitch adjustment system. If the turbine is idling, the generator torque reference 903 is zero, and the collective pitch reference 904 is constant. Thus, the zero generator torque references and the constant pitch reference are used for controlling the wind turbine so as to be in an idling power producing situation in relation to the utility grid. Alternatively, or in addition, the zero generator torque references and the constant pitch reference can be used for a detection that the wind turbine is in an idling power producing situation in relation to the utility grid.

(10) Thus, by means of the constant pitch reference the speed controller 901 is capable of maintaining a rotation speed of the rotor 5 during idling within an acceptable rotation speed range. For example, the constant pitch reference may be set to obtain a rotation speed within a range of 10-25 percent of the nominal rotation speed, i.e. the maximum rotation speed used during full load operation. When the rotation speed of the rotor is within this range, excitation of structural oscillations, e.g. tower oscillations, due to the rotation of the rotor may be avoided or reduced. At greater rotation speeds, e.g. above 30 percent of the nominal rotation speed, the rotor may excite tower oscillations.

(11) In an embodiment the speed controller 901 is configured for controlling the rotation speed of the rotor by adjusting the pitch angle of the at least one rotor blade. FIG. 3 shows an example of the speed controller 901 configured as a feedback controller, e.g. a PID controller, which determines the pitch angle of the at least one rotor blade, i.e. the collective pitch reference 904, based on a control algorithm, a received rotor speed reference 301, a measured rotor speed 302, and optionally the input data 902. Accordingly, based on difference between the rotor speed reference 301, i.e. the desired rotor speed, and the measured rotor speed, the control algorithm of the speed controller 901 determines the collective pitch reference 904 so that the difference is minimised, i.e. so that the rotor speed approaches the rotor speed reference 301.

(12) The rotor speed reference 301 may have a value so that the rotation speed of the rotor is controlled to be within a range from 10 to 25 percent of the nominal rotation speed of the rotor. For example the rotor speed reference may be set to 15 percent of the nominal rotation speed of the rotor.

(13) FIG. 4 illustrates how the rotational speed 401 of the rotor 5 is controlled dependent on wind speed data provided by input 902 and how the generated power 402 varies with the wind speed. At wind speeds between v_in and v_rtd the wind turbine is operated in a partial load mode, at wind speeds between v_rtd and v_derate the wind turbine is operated in a full load mode and at wind speeds between v_derate and v_off the wind turbine is operated in a de-rate mode. When the wind speed increases to v_off the wind turbine is operated in the idling mode according to embodiments of this invention wherein the rotor speed is reduced and power production from the generator is zero.

(14) Referring to FIG. 2, based on input 905 from the acceleration sensor 10, the controller detects a longitudinal tower oscillation, and provides, with a longitudinal damper unit 906, based on the input 905, a collective pitch reference offset 907 which is added to the collective pitch reference 904. With the collective pitch reference offset 907 the pitch angles of each of the rotor blades are controlled 908, 909, 910 so as to produce aerodynamic forces counteracting the detected longitudinal tower oscillation.

(15) For lateral oscillation, based on input 911 from the acceleration sensor 10, the controller detects a lateral tower oscillation, and provides, with a lateral damper unit 912, based on the input 911, an individual cyclic pitch reference offset 913, 914, 915 for each blade, which is added to the collective pitch reference 904. The cyclic effect can be obtained using a function of the current position of the rotor (azimuth position). With the cyclic pitch reference offset 913, 914, 915 the pitch angles of each of the rotor blades are controlled 908, 909, 910 so as to produce aerodynamic forces counteracting the detected lateral tower oscillation.

(16) The determination of the cyclic pitch reference offset 913, 914, 915 may be performed by integrating the acceleration signal from input 911 to obtain a velocity signal of the lateral tower oscillation. The velocity signal may be multiplied by a feedback gain to obtain a modulation signal. The modulation signal is phase shifted, e.g. multiplied with minus one, in order to create a damping signal which can be used for creating forces via individual pitch actuation for counteracting the lateral tower oscillation. The cyclic pitch reference offsets 913, 914, 915 can be obtained from the damping signal by multiplying the damping signal with a sinusoidal function for each of the offsets. The sinusoidal functions are in the form sin(t+i), where is the rotation frequency of the rotor and i represents an angular position of the blade i. Accordingly, for a rotor with three blades, three cyclic pitch reference offsets 913, 914, 915 are determined where, as an example, i=[0, 2/3, 2/3] for i=1,2,3.

(17) The determination of the collective pitch reference offset 907 may be performed similarly by integrating the acceleration signal from input 905 to obtain a velocity signal of the longitudinal tower oscillation. The velocity signal may be multiplied by a feedback gain to obtain a modulation signal. The modulation signal is phase shifted, e.g. multiplied with minus one, in order to create a damping signal which can be used for creating forces via collective pitch actuation for counteracting the longitudinal tower oscillation.

(18) In case a combination of longitudinal and lateral oscillations are detected, the collective pitch reference offset 907 from the longitudinal damper unit 906 is added to the cyclic pitch reference offsets 913, 914, 915 from the lateral damper unit 912.

(19) FIG. 5 shows tower oscillations 501 corresponding to a measurement signal from an acceleration sensor 10 during an idling power producing situation. The tower oscillations may be in the form of longitudinal oscillations, lateral oscillations or a combination thereof. Initially, up to time t1, the tower oscillations are not damped actively by the damper units 906, 912. After t1 one or both of the damper units starts generating pitch reference offsets 907, 913, 914, 915. Due to the phase of the pitch reference offsets, the pitch action generates an oscillating damping force 502 which is in phase with the velocity signal (multiplied by minus one) of the tower oscillations and, therefore, causes damping of the tower oscillations. The amplitude of the damping force oscillations 502 and, thereby, the amplitude of the pitch reference offset oscillations may vary, e.g. as shown with an initial increase.

(20) The damper units 906, 912 may be configured as feedback damper units which determines the pitch reference offsets 907, 913, 914, 915 as a function of the difference between the oscillation amplitudes provided via input 905,911 and a reference amplitude, e.g. zero, which represents the desired maximum tower oscillation amplitude. Accordingly, as the measured oscillation amplitude approaches the reference amplitude, the oscillating damping force 502 decreases as illustrated by the portion of the damping force 502 with decreasing amplitude.

(21) Referring to FIG. 1, the power to the pitch adjustment system for controlling the pitch angle can be received from the utility grid, if this is available, or from an auxiliary power source 11. This auxiliary power source can be of any suitable kind, e.g. an electric power storage unit with batteries, or a backup diesel generator unit.