A METHOD FOR HANDLING ROTOR UNBALANCE OF A WIND TURBINE WITH HINGED WIND TURBINE BLADES

Abstract

A method for operating a wind turbine with hinged wind turbine blades is disclosed. The wind turbine comprises an adjustable biasing mechanism arranged to apply an adjustable biasing force to each wind turbine blade which biases the wind turbine blade towards a position defining a minimum pivot angle or towards a position defining maximum pivot angle. A biasing force is selected for each wind turbine blade and the selected biasing force is applied to the respective wind turbine blades. The wind turbine is operated while monitoring rotor unbalance of the wind turbine. In the case that the rotor unbalance exceeds a first threshold value at least one of the wind turbine blades is selected, and the biasing force applied to the selected wind turbine blade(s) is adjusted.

Claims

1. A method for operating a wind turbine, the wind turbine comprising a tower, a nacelle mounted on the tower via a yaw system, a hub mounted rotatably on the nacelle, the hub comprising a blade carrying structure, and one or more wind turbine blades connected to the blade carrying structure via a hinge, each wind turbine blade thereby being arranged to perform pivot movements relative to the blade carrying structure between a minimum pivot angle and a maximum pivot angle, wherein the pivot angle determines a diameter of a rotor of the wind turbine, the wind turbine further comprising an adjustable biasing mechanism arranged to apply an adjustable biasing force to each wind turbine blade which biases the wind turbine blade towards a position defining minimum pivot angle or towards a position defining maximum pivot angle, the method comprising: selecting a biasing force for each wind turbine blade and applying the selected biasing force to the respective wind turbine blades; operating the wind turbine while monitoring rotor unbalance of the wind turbine; and in the case that the rotor unbalance exceeds a first threshold value: selecting at least one of the wind turbine blades; and adjusting the biasing force applied to the selected wind turbine blade(s).

2. The method of claim 1, wherein the selected biasing force is applied as a set-point for a desired biasing force for the wind turbine blades.

3. The method of claim 1, wherein the adjustable biasing mechanism is arranged so that the pivot angle of each wind turbine blade is a result of a balancing between at least the biasing force and a wind load force acting on the respective wind turbine blade.

4. The method of claim 1, wherein the monitoring rotor unbalance of the wind turbine comprises monitoring the pivot angle of each wind turbine blade.

5. The method of claim 4, further comprising determining that the rotor unbalance exceeds the first threshold value in the case that a difference in pivot angle between at least two wind turbine blades exceeds a predefined threshold value.

6. The method of claim 4, wherein the selecting at least one wind turbine blade comprises selecting the wind turbine blade having a pivot angle which differs most from the pivot angles of any of the other wind turbine blades.

7. The method of claim 1, wherein the monitoring rotor unbalance of the wind turbine comprises monitoring accelerations of the nacelle and/or an upper part of the tower along at least one direction.

8. The method of claim 1, wherein the adjusting the biasing force applied to the selected wind turbine blade(s) comprises adjusting a pulling force applied to a wire connected to each selected wind turbine blade.

9. The method of claim 1, further comprising after adjusting the biasing force applied to the selected wind turbine blade(s), monitoring the rotor unbalance of the wind turbine; in the case that the rotor unbalance decreases, adjusting the biasing force applied to the selected wind turbine blade(s) further; and in the case that the rotor unbalance increases, reversing the adjustment of the biasing force applied to the selected wind turbine blade(s).

10. The method of claim 1, further comprising: after adjusting the biasing force applied to the selected wind turbine blade(s) (5), monitoring the rotor unbalance of the wind turbine; in the case that the rotor unbalance increases or remains unchanged, selecting at least one further wind turbine blade; and adjusting the biasing force applied to the further wind turbine blade(s).

11. The method of claim 1, further comprising stopping the wind turbine in the case that the rotor unbalance exceeds a second threshold value, the second threshold value being larger than the first threshold value.

12. The method of claim 1, wherein the first threshold value is variable as a function of ambient temperature and/or humidity.

13. The method of claim 1, wherein the adjusting the biasing force applied to the selected wind turbine blade(s) is performed based on ambient temperature and/or humidity.

14. A wind turbine, comprising: a tower; a nacelle mounted on the tower via a yaw system; a hub mounted rotatably on the nacelle, the hub comprising a blade carrying structure; one or more wind turbine blades connected to the blade carrying structure via a hinge, each wind turbine blade thereby being arranged to perform pivot movements relative to the blade carrying structure between a minimum pivot angle and a maximum pivot angle, wherein the pivot angle determines a diameter of a rotor of the wind turbine; an adjustable biasing mechanism arranged to apply an adjustable biasing force to each wind turbine blade which biases the wind turbine blade towards a position defining minimum pivot angle or towards a position defining maximum pivot angle; and a controller configured to perform an operation, comprising: selecting a biasing force for each wind turbine blade and applying the selected biasing force to the respective wind turbine blades; operating the wind turbine while monitoring rotor unbalance of the wind turbine; and upon detecting that the rotor unbalance exceeds a first threshold value: selecting at least one of the wind turbine blades; and adjusting the biasing force applied to the selected wind turbine blade.

15. The wind turbine of claim 14, wherein the selected biasing force is applied as a set-point for a desired biasing force for the wind turbine blades.

16. The wind turbine of claim 14, wherein the adjustable biasing mechanism is arranged so that the pivot angle of each wind turbine blade is a result of a balancing between at least the biasing force and a wind load force acting on the respective wind turbine blade.

17. The wind turbine of claim 14, wherein the monitoring rotor unbalance of the wind turbine comprises monitoring the pivot angle of each wind turbine blade.

18. The wind turbine of claim 17, the operation further comprising determining that the rotor unbalance exceeds the first threshold value in the case that a difference in pivot angle between at least two wind turbine blades exceeds a predefined threshold value.

19. The wind turbine of claim 17, wherein the selecting at least one wind turbine blade comprises selecting the wind turbine blade having a pivot angle which differs most from the pivot angles of any of the other wind turbine blades.

20. The wind turbine of claim 14, wherein the monitoring rotor unbalance of the wind turbine comprises monitoring accelerations of the nacelle and/or an upper part of the tower along at least one direction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0078] The invention will now be described in further detail with reference to the accompanying drawings in which

[0079] FIGS. 1-3 illustrate a wind turbine being controlled in accordance with a method according to a first embodiment of the invention,

[0080] FIGS. 4-6 illustrate a wind turbine being controlled in accordance with a method according to a second embodiment of the invention,

[0081] FIGS. 7 and 8 show details of a biasing mechanism for use in a method according to an embodiment of the invention,

[0082] FIG. 9 is a block diagram illustrating a method according to an embodiment of the invention, and

[0083] FIG. 10 is a flow chart u rating a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0084] FIGS. 1-3 illustrate a wind turbine 1 being controlled in accordance with a method according to a first embodiment of the invention. FIG. 1 is a front view of the wind turbine 1, and FIGS. 2 and 3 are side views of the wind turbine 1.

[0085] The wind turbine 1 of FIGS. 1-3 comprises a tower 2 and a nacelle 7 mounted on the tower 2. A hub 3 is mounted rotatably on the nacelle 7, the hub 3 comprising a blade carrying structure 4 with three arms. A wind turbine blade 5 is connected to each of the arms of the blade carrying structure 4 via a hinge 6. Thus, the wind turbine blades 5 rotate along with the hub 3, relative to the nacelle 7, and the wind turbine blades 5 can perform pivoting movements relative to the blade carrying structure 4, via the hinges 6.

[0086] Each wind turbine blade 5 defines an aerodynamic profile extending along the length of the wind turbine blade 5 between an inner tip end 5a and an outer tip end 5b, The hinge 6 is arranged at a hinge position of the wind turbine blade 5, the hinge position being at a distance from the inner tip end 5a as well as at a distance from the outer tip end 5b. The wind turbine blades 5 of the wind turbine 1 of FIGS. 1-3 are straight in the sense that an inner portion of the wind turbine blade 5, between the hinge 6 and the inner tip end 5a, and an outer portion of the wind turbine blade 5, between the hinge 6 and the outer tip end 5b, extend along the same direction, i.e. an angle is not formed between the inner and outer portions of the wind turbine blade 5.

[0087] A biasing mechanism comprising wires 8 attached to the wind turbine blades S at a position near the inner tip end 5a applies a biasing force to the wind turbine blades 5 which pulls the wind turbine blades 5 towards a position defining minimum pivot angle, and thereby maximum rotor diameter. This will be described in further detail below with reference to FIGS. 7 and 8.

[0088] In FIG. 2 the wind turbine blades 5 are positioned at the minimum pivot angle, i.e. at a pivot angle which results in a maximum rotor diameter of the wind turbine 1.

[0089] In FIG. 3 the wind turbine blades 5 are positioned at a larger pivot angle P than the minimum pivot angle illustrated in FIG. 2. Thereby the rotor diameter of the wind turbine 1 is smaller in the situation illustrated in FIG. 3 than in the situation illustrated in FIG. 2.

[0090] The wind turbine 1 of FIGS. 1-3 is a downwind wind turbine, i.e. the wind direction relative to the wind turbine 1 is illustrated by arrow 9 shown in FIGS. 2 and 3.

[0091] The wind turbine 1 of FIGS. 1-3 may be operated in the following manner. Initially, a biasing force is selected for each of the wind turbine blades 5, and the biasing mechanism 8 is adjusted to apply the selected biasing force to the respective wind turbine blade 5. Thereby the wind turbine blades 5 are pulled towards the position shown in FIG. 2.

[0092] Next, the wind turbine 1 is operated, while rotor unbalance of the wind turbine 1 is monitored. This could, e.g., include monitoring the pivot angles of the wind turbine blades 5, accelerations of the nacelle 7 and/or an upper part of the tower 2, deflections of the nacelle 7 and/or the upper part of the tower 2, torque on a main shaft and/or a generator shaft, etc., as described above.

[0093] In the case that the rotor unbalance exceeds a first threshold value, at least one of the wind turbine blades 5 is selected. For instance, a wind turbine blade 5 having a pivot angle which differs significantly from the pivot angles of the other wind turbine blades 5 may be selected. The biasing force applied to the selected wind turbine blade(s) is then adjusted in order to attempt to eliminate, or at least reduce, the rotor unbalance. This may include increasing or reducing the biasing force, depending on the nature of the detected rotor unbalance.

[0094] If the adjustment of the biasing force applied to the selected wind turbine blade(s) causes a reduction in the rotor unbalance, further adjustment of the biasing force applied to the selected wind turbine blade(s), in the same direction, may be performed until the rotor unbalance has been eliminated. On the other hand, if the adjustment of the biasing force causes an increase in the rotor unbalance, the performed adjustment may be reversed and/or one of the other wind turbine blades 5 may be selected, and the biasing force applied to this wind turbine blade 5 may be adjusted.

[0095] FIGS. 4-6 illustrate a wind turbine 1 being controlled in accordance with a method according to a second embodiment of the invention. The wind turbine 1 of FIGS. 4-6 is very similar to the wind turbine 1 of FIGS. 1-3, and will therefore not be described in detail here. FIGS. 4-6 are all side views of the wind turbine 1 with the wind turbine blades 5 arranged at three different pivot angles. FIG. 4 shows the wind turbine blades 5 at minimum pivot angle, FIG. 6 shows the wind turbine blades 5 at maximum pivot angle, or barrel mode, and FIG. 5 shows the wind turbine blades 5 at an intermediate pivot angle.

[0096] The wind turbine blades 5 of the wind turbine 1 of FIGS. 4-6 are angled in the sense that the inner portion and the outer portion of the wind turbine blade 5 extend from the hinge 6 along different directions, forming an angle there between.

[0097] FIGS. 7 and 8 show details of a biasing mechanism for applying a biasing force to wind turbine blades 5 of a wind turbine, e.g. the wind turbine 1 of FIGS. 1-3 or the wind turbine 1 of FIGS. 4-6.

[0098] FIG. 7 shows a portion of a blade carrying structure 4 and a portion of a wind turbine blade 5. The wind turbine blade 5 is pivotally mounted on the blade carrying structure 4 via a hinge (not shown). A wire 8 is connected to the wind turbine blade 5 at a position between an inner tip end 5a of the wind turbine blade 5 and the position of the hinge. The wire 8 extends from the connecting position at the wind turbine blade 5, via a pulley 10 and along the blade carrying structure 4 towards a hub (not shown).

[0099] A biasing force applied by means of the wire 8 pulls the wind turbine blade 5 towards a position defining a minimum pivot angle. In FIG. 7 the wind turbine blade 5 is arranged at the minimum pivot angle. Reducing the biasing force applied by means of the wire 8 will allow the wind turbine blade 5 to more easily pivot towards larger pivot angles.

[0100] FIG. 8 is a cross sectional view of part of a hub 3 and part of a nacelle 7. Arms of a blade carrying structure 4 are mounted on the hub 3. The wires 8 which are also illustrated in FIG. 7 are connected to a winch mechanism 11 arranged in the hub 3. Thereby the biasing force applied by means of the wires 8 can be adjusted by rotating the winch mechanism 11, thereby adjusting the length of the wires 8.

[0101] FIG. 9 is a block diagram illustrating a method according to an embodiment of the invention. Pivot angles, Piv1, Piv2 and Piv3, for three wind turbine blades are measured and supplied to a bandpass filter block 12. In the bandpass filter block 12, the measured pivot angles are analysed in order to derive the frequency content of the measured values, in particular information regarding 1P frequencies. Based on this analysis, the bandpass filter block 12 outputs a signal indicating that a rotor unbalance is present, and transmits this to a stiffness correction block 13.

[0102] In response thereto, and based on a set of evaluation parameters, the stiffness correction block 13 selects at least one of the wind turbine blades and adjusts a biasing force setpoint for the selected wind turbine blade(s). Finally, the stiffness correction block 13 transmits biasing force setpoints to a pivot control unit 14, and the pivot control unit 14 controls the biasing mechanism in accordance with the received setpoints.

[0103] In the embodiment illustrated in FIG. 9, the biasing mechanism is of a kind which comprises hydraulic mechanisms, and the biasing force setpoints are in the form of pressure setpoints for the hydraulic mechanisms.

[0104] FIG. 10 is a flow chart illustrating a method according to an embodiment of the invention. The process is started at step 15. At step 16, biasing forces are selected for each wind turbine blade, and the selected biasing forces are applied to the respective wind turbine blades.

[0105] At step 17 rotor unbalance of the wind turbine is monitored, and it is investigated whether or not the rotor unbalance is above a first threshold. This could, e.g., include monitoring pivot angles, accelerations and/or deflections of the nacelle and/or an upper part of the tower, torque on the main shaft and/or the generator shaft, etc., as described above.

[0106] In the case that step 17 reveals that the rotor unbalance is not above the first threshold, the wind turbine continues operating in a normal manner, and monitoring of the rotor unbalance is continued.

[0107] In the case that step 17 reveals that the rotor unbalance exceeds the first threshold value, it is concluded that the rotor unbalance is at a level which requires action. Therefore, the process is forwarded to step 18, where at least one of the wind turbine blades is selected, and the biasing force applied to the selected wind turbine blade is adjusted, in order to attempt to counteract the rotor unbalance. For instance, the selected wind turbine blade may be a wind turbine blade which somehow differs from the other wind turbine blades, e.g. a wind turbine blade with a pivot angle, pivot angle movement pattern, or similar, which differs significantly from that of the other wind turbine blades. As an alternative, a random wind turbine blade may be selected.

[0108] At step 19 it is investigated whether or not the performed adjustment of the biasing force has caused a reduction in the rotor unbalance. If this is the case, it can be concluded that the selected strategy is effective, and the process is therefore forwarded to step 20, where the biasing force of the selected wind turbine blade is adjusted further in the same direction, in order to further reduce the rotor unbalance. Subsequently, the process is returned to step 17 in order to investigate whether or not the rotor unbalance has been reduced to a level below the first threshold.

[0109] In the case that step 19 reveals that the rotor unbalance has not been reduced, it can be concluded that the performed adjustment of the biasing force has been non-affective to the rotor unbalance, or it might even have increased the rotor unbalance, thereby making the problem worse instead of solving it. The biasing force should therefore not be adjusted further in the same manner. Instead, the process is forwarded to step 21, where it is investigated whether or not the rotor unbalance has increased as a result of the performed adjustment of the biasing force. If this is not the case, it can be concluded that the performed adjustment of the biasing force had no effect on the rotor unbalance, i.e. it has not solved the problem, but it has neither made the problem worse.

[0110] Therefore, in this case the process is returned to step 18, where another wind turbine blade is selected, and the procedure described above is performed for this wind turbine blade.

[0111] If step 21 reveals that the rotor unbalance has increased as a result of the adjustment of the biasing force, it can be concluded that the performed adjustment of the biasing force has made the problem worse instead of solving it, and further measures are therefore required. Accordingly, the process is forwarded to step 22, where it is investigated whether or not the rotor unbalance has increased to a level above a second threshold, which is higher than the first threshold. If this is the case, this may be an indication that there is an immediate risk of damage to the wind turbine, Therefore, in this case the process is forwarded to step 23, where the wind turbine is stopped. Furthermore, a de-icing procedure may be initiated and/or a service team may be ordered.

[0112] In the case that step 22 reveals that the rotor unbalance is not above the second threshold, the process is forwarded to step 24, where the performed adjustments to the biasing force are reversed, before the process is returned to step 18, where another wind turbine blade is selected.