Control system and method for operating a plurality of wind turbines

Abstract

A method for operating a first wind turbine and a second wind turbine, the second wind turbine being located in the wake of the first wind turbine. A prediction model is fed with a current wind value of the first wind turbine, in order to predict a future time point at which the area swept by the rotor of the second wind turbine becomes partially overlapped by the wake of the first wind turbine. The second wind turbine reacts to the prediction in that a control signal is generated in order to alter the pitch angle of a rotor blade of the second wind turbine relative to the pitch angle of another rotor blade of the second wind turbine. The invention additionally relates to a control system suitable for executing the method. Implementation of the disclosed method by a control system can reduce the loading of the second wind turbine.

Claims

1. A method for operating a first wind turbine (14) and a second wind turbine (15), the second wind turbine (15) being located downwind of the first wind turbine (14), said method comprising: providing a current wind value of the first wind turbine (14) to a prediction model (28), said current wind value comprising a current wind speed value or a current wind direction value; in said prediction model (28), using the current wind value of the first wind turbine (14) to predict a future time point (25) at which an area swept by a rotor of the second wind turbine (15) overlaps a wake (30) of the first wind turbine (14); in response to said prediction, generating a control signal to alter a pitch angle of a rotor blade of the second wind turbine (15) relative to the pitch angle of another rotor blade of the second wind turbine (15) before the future time point (25); and beginning to adapt the pitch angle of the rotor blades of the second wind turbine (15) to the control signal before the future time point.

2. The method of claim 1, wherein the prediction model (28) comprises an assumption that the wake (30) widens in a radial direction as a distance from the first wind turbine (14) increases.

3. The method of claim 1, wherein the prediction model (28) comprises an assumption that a wind speed in the wake (30) is reduced in comparison with a wind field that is not disturbed by the first wind turbine (14).

4. The method of claim 1, wherein the prediction model (28) comprises an assumption that the wind speed in the wake (30) has a radial distribution according to which a wind speed in the center of the wake (30) is less than in the periphery of the wake (30).

5. The method of claim 1, wherein the prediction model (28) comprises an assumption that the turbulence intensity in the wake (30) is increased in comparison with a wind field that is not disturbed by the first wind turbine (14).

6. The method of claim 1, comprising determining an estimated wind value on the basis of an air mass-flow acting upon the entire rotor of the first wind turbine (14); and using the estimated wind value as the current wind value of the first wind turbine (14).

7. The method of claim 1, wherein the current wind value of the first wind turbine (14) and a current wind value the second wind turbine (15) are used to predict a propagation direction of the wake (30).

8. The method of claim 1, wherein the prediction model maps a wind prediction into a geographical coordinate system.

9. The method of claim 8, wherein a location of the first wind turbine (14) and the location of the second wind turbine (15) are defined by coordinates within the geographical coordinate system.

10. The method of claim 1, wherein the current wind value comprises a value for turbulence intensity derived from a standard deviation of a current output power of the first wind turbine (14).

11. The method of claim 1, wherein the prediction model (28) determines a time span within which a change in wind direction propagates from the first wind turbine (14) to the second wind turbine (15) as the sum of a time lag, which is derived from the current wind speed value and a wake-related additional time.

12. The method of claim 1, comprising: equipping the second wind turbine (15) with load sensors to determine an actual wind load acting on the second wind turbine (15).

13. The method of claim 1, comprising: determining a sequence of current wind values at different time points for the first wind turbine (14) or the second wind turbine (15); and storing the sequence of current wind values for the first wind turbine (14) or the second wind turbine (15).

14. The method of claim 1, wherein the pitch angle of the rotor blades of the second wind turbine (15) is adapted to the control signal before the future time point.

15. A control system for wind turbines, said control system comprising: an acquisition means for recording a current wind value of a first wind turbine (14), a prediction model (28) that processes the current wind value to predict a future time point (25) at which a second wind turbine (15), located in a wake (30) of the first wind turbine (14), is overlapped by the wake (30) of the first wind turbine (14), wherein, in response to said prediction, the control system generates a control signal to alter a pitch angle of a rotor blade of the second wind turbine (15) relative to a pitch angle of another rotor blade of the second wind turbine (15) before the future time point (25) and begins to adapt the pitch angle of the rotor blades of the second wind turbine (15) to the control signal before the future time point.

16. The control system of claim 15, wherein the pitch angle of the rotor blades of the second wind turbine (15) is adapted before the future time point (25).

17. The control system of claim 16, wherein the pitch angle of the rotor blades of the second wind turbine according to the control signal varies depending upon the angular position of each rotor blade.

18. A method for operating a first wind turbine (14) and a second wind turbine (15), the second wind turbine (15) being located downwind of the first wind turbine (14), said method comprising: when a current wind speed at said first turbine (14) is above a nominal speed or the second wind turbine (15) is presently being operated at a nominal output; providing a current wind value of the first wind turbine (14) to a prediction model (28), said current wind value comprising a current wind speed value or a current wind direction value; in said prediction model (28), using the current wind value of the first wind turbine (14) to predict a future time point (25) at which an area swept by a rotor of the second wind turbine (15) overlaps a wake (30) of the first wind turbine (14); and in response to said prediction, generating a control signal to alter a pitch angle of a rotor blade of the second wind turbine (15) relative to the pitch angle of another rotor blade of the second wind turbine (15).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is described exemplarily in the following with reference to the appended drawings, on the basis of advantageous embodiments. There are shown:

(2) FIG. 1: an arrangement of wind turbines according to the invention;

(3) FIG. 2: a schematic representation of a prediction model according to the invention;

(4) FIG. 3: a partial overlap between the wake of a first wind turbine and the rotor of a second wind turbine;

(5) FIG. 4: an aspect of the prediction model according to the invention;

(6) FIG. 5: an arrangement of wind turbines according to the invention; and

(7) FIG. 6: a flow chart showing an example of steps in applying a prediction model according to aspects of the invention.

DETAILED DESCRIPTION

(8) An arrangement of wind turbines shown in FIG. 1 comprises a first wind turbine 14, a second wind turbine 15 and a third wind turbine 16. Each wind turbine 14, 15, 16 comprises a rotor, which is put into rotation by the wind and which drives a generator for the purpose of generating electrical energy. Each of the wind turbines 14, 15, 16 additionally comprises a wind estimator 17, which in FIG. 1 is represented only on the example of the wind turbine 15.

(9) The wind estimators 17 are designed to determine, for each of the wind turbines 14, 15, 16, an estimated value for the wind speed and possibly also for the wind direction. Each estimated value is derived from one or more operating parameters, which change in dependence on the air-mass flow acting upon the entire rotor. These operating parameters may comprise, for example, the rotational speed of the rotor, the pitch angle of the rotor blades and the torque applied by the rotor. Since the rotor sweeps a large area, reliable values for the wind speed, and optionally also those for the wind direction, can be estimated by means of the wind estimator 17. The latter requires additional measurement values, preferably, for example, a measurement of the individual blade motion or deformation, or a measurement of the tower motion or deformation.

(10) According to aspects of the invention, a control system implementing the disclosed prediction model 28 includes acquisition means for recording a current wind value of a first wind turbine (14). Acquisition means includes sensors and measurements of the condition and operating parameters of a wind turbine, a system for communicating this data to a computer, and calculations performed on the sensor readings and measured values to determine a current estimated wind value for a wind turbine at a time point. A wind turbine 15 may be equipped with load sensors 11 to determine an actual wind load acting on the wind turbine 15. A current estimated wind value may be used in the prediction model as the current wind value according to aspects of the invention.

(11) In the case of the wind direction 18 prevailing in FIG. 1, the second wind turbine 15 is located downwind from the first wind turbine 14. The distance between the second wind turbine 15 and the first wind turbine 14, as viewed in the wind direction, is such that the second wind turbine 15 is located in the wake 30 of the first wind turbine 14. Thus, in the case of the wind direction 18, the wind conditions experienced by the second wind turbine 15 are influenced by the operation of the first wind turbine 14.

(12) Although the third wind turbine 16 is also located upwind of the second wind turbine 15, the third wind turbine 16 is offset so far to the side that there is no intersection between the wake 30 of the third wind turbine 16 and the rotor of the second wind turbine 15. At the start of the method according to the invention, it is ascertained, taking into account the current wind direction, which wind turbines are located in the wake of which other wind turbines. In the exemplary embodiment according to FIG. 1, only the second wind turbine 15, located in the wake of the first wind turbine 14, is affected.

(13) Estimated wind values of all three wind turbines 14, 15, 16 are determined, at the current time point 20, on the time axis 19 in FIG. 1. The estimated wind values are passed to a prediction computer 21, in which there is a stored prediction model 28. The estimated wind values are fed into the prediction model 28, which processes the estimated wind values in order to predict how the wake 30 of the first wind turbine 14 will spread.

(14) A propagation direction and a propagation speed of the wake are determined by means of the prediction model 28. In the example according to FIG. 1, the propagation direction is derived from the current wind direction 18, and the propagation speed from the wind speed. In FIG. 1 this is represented schematically by a front line 22, which is aligned parallel to the rotor of the first wind turbine 14 and which moves downwind. A mean value of the wind direction estimated in the case of the wind turbines 14, 16 is assumed as a current wind direction. The flow received by both wind turbines 14, 16 is unobstructed, such that the wind direction is not falsified by the wake of a wind turbine located farther to the front. In other embodiments of the invention, an estimated value obtained in the case of the second wind turbine 15 may also be taken into account in the determination of the current wind direction. A corresponding procedure may be applied in determining the current wind speed.

(15) A control unit 24 of the second wind turbine 15 may request, from the prediction computer 21, how the wake 30 of the first wind turbine 14 is likely to affect the second wind turbine 15 at the future time point 25. In the exemplary embodiment according to FIG. 1, at the future time point 25 there is a partial overlap between the wake 30 of the first wind turbine 14 and the rotor of the second wind turbine 15.

(16) As a reaction to this prediction, shortly before the time point 25 the control unit 24 generates a control signal, according to which the pitch angle of the rotor blades of the second wind turbine 15 is adapted, before the time point 25, in dependence on the angular position of the respective rotor blade (pre-control). In particular, in the angular range of the rotor revolution in which there is an intersection with the wake 30 of the first wind turbine 14, each rotor blade may have a pitch angle that differs from that in other angular ranges of the rotor revolution. The control signal of the control unit 24 can thus effect a cyclic alteration of the pitch angle, and the control signals can be repeated after their complete rotor revolution.

(17) A current value of the turbulence intensity may additionally be determined at the current time point 20. The turbulence intensity may be determined, for example, from the standard deviation of the output power delivered by the first wind turbine 14. The prediction computer 21 may be designed such that it also determines a propagation of the turbulence intensity, and makes a corresponding prediction for the future time point 25. The control specification of the control unit 24 for the second wind turbine 15 may also depend on the turbulence intensity predicted for the time point 25.

(18) Illustrated in FIG. 2 are some assumptions on which the prediction model 28 for the wake is based. If constant wind conditions are assumed, the wake 30 propagates rearwards, behind the rotor blade of the first wind turbine 14, concentrically in relation to the rotor axis. In the region of the first wind turbine 14, the cross section of the wake 30 corresponds to the area swept by the rotor. As the distance from the first wind turbine 14 increases, the wake 30 widens slightly in the radial direction.

(19) In the region of the wake 30 the wind speed is less than it would be in a wind field not disturbed by the first wind turbine 14. The wind speed deficit is greatest in the region directly adjoining the rotor (with dark coloring). As the distance from the first wind turbine 14 increases in the downwind direction, and as the distance from the central axis of the wake 30, represented by a broken line, increases, the wind speed deficit becomes less. Accordingly, in FIG. 2, the second wind turbine 15 is affected only by a slight wind speed deficit, in the edge region of the wake 30.

(20) In FIG. 3, the area swept by the rotor of the second wind turbine 15 is identified by a circle 22. According to the prediction model 28, the wake 30 likewise has a circular cross section in the plane of the rotor of the second wind turbine 15. Between the wake 30 and the area 22 swept by the rotor there is a partial overlap, in the region 23.

(21) FIG. 4 shows an exemplary possibility of how changes in the wind direction can be mapped in the prediction model 28. Represented is a period between a current time point t and a past time point t3t. At the past time point t3t the wind came from 270, such that the second wind turbine 15 was located entirely in the wake 30 of the first wind turbine 14. Between t3t and t2t the wind direction turned by 10, to 260. Owing to the wake 30, there is a resultant delay until this change in the wind direction reaches the second wind turbine 15. Between t2t and tt the wind turns by a further 10, to 250, and maintains this new wind direction up to the current time point t. The graphic indicates the respective delay until the changed wind direction has propagated into the wake 30.

(22) In the case of the wind park represented schematically in FIG. 5, the wind turbines located upwind, as first wind turbines 14, are represented separately from the second wind turbines 15 located downwind. The basic pair-wise allocation between first wind turbines 14 and second wind turbines 15 is dependent on the wind direction, and must be adapted upon each change in the wind direction. All wind turbines 14, 15 are equipped with wind estimators 17, which send estimated wind values to a central buffer 33. In the central buffer 33, the estimated wind values are stored together with the geographical coordinates of the respective wind turbine and with an associated time stamp. In addition to this, in each case information may be stored about the operating state of the wind turbine concerned at the respective time point.

(23) The data are stored in the buffer 33 for as long as is required by the slowest wind field to be considered to move over the greatest extent of the wind park. The slowest wind field to be considered may have a speed, for example, corresponding to the cut-in wind speed of the wind turbines. The buffer 33 is realized as a ring buffer, such that the oldest data of this period are in each case replaced by newer data.

(24) Each wind turbine 14, 15 is equipped with a decentralized prediction computer 35, which can access the estimated wind values in the buffer 33. For each second wind turbine 15 located downwind, a geographical sector, in which a first wind turbine 14 could generate a disturbing wake, is obtained from the current wind direction. By requesting the geographical coordinates of the first wind turbines 14 that are stored in the buffer 33, the prediction computer can identify individual wind turbines 14 that potentially could cause a disturbing wake. As a result of the prediction computer 35 accessing the current estimated wind values of the respective first wind turbine in the buffer 33, the prediction computer can predict whether a disturbing wake 30 is soon to be anticipated for its own wind turbine 15. If this is the case, shortly before the arrival of the disturbing wake the wind turbine 15 can generate a control signal, according to which the rotor blades are pitched cyclically.

(25) With reference to FIG. 6, the method step of applying the prediction model is explained in more detail. The method starts in step 100. In step 110 a current value of the wind speed and a current value of the wind direction at the first wind turbine 14 are obtained.

(26) In step 120 a wake 30 of the first wind turbine 14 is determined so that a central axis 36 of the wake is coaxial to the rotor axis of the first wind turbine 14 and so that the wake 30 is circular in cross-section. The wake 30 may have a length 39 that corresponds to four times the rotor diameter of the first wind turbine 14.

(27) In one embodiment the wake 30 has a constant diameter over its length 39, wherein the diameter corresponds to the diameter of the rotor of the first wind turbine 14. In another embodiment the diameter of the wake 30 increases with increasing distance from the first wind turbine 14.

(28) In step 130 an axial distance 38 between the first wind turbine 14 and the second wind turbine 15 is determined. In step 140 a radial distance 37 between the central axis 36 of the wake 30 and the rotor axis of the second wind turbine 15 is determined.

(29) In step 150 it is determined whether there is an overlap between the wake 30 of the first wind turbine 14 and the rotor of the second wind turbine 15. This is done by comparing the length 39 of the wake 30 with the axial distance 38 between the first wind turbine 14 and the second wind turbine 15 and by comparing the radial distance 37 with the radius of the rotor of the second wind turbine 15 and with the radius of the wake 30. If the axial distance 38 is smaller than the length 39 of the wake 30 and if the radial distance 37 is smaller than the sum of the radius of the rotor of the second wind turbine 15 and the radius of the wake 30 at the same axial position, there is an overlap between the wake 30 and the rotor of the second wind turbine 15. If one of the two conditions is not met there is no overlap between the wake 30 and the rotor of the second wind turbine 15. In this case the method starts over with step 100.

(30) If there is an overlap the method proceeds to step 160. In step 160 a future time point 25 is determined when the second wind turbine 15 is hit by the currently determined wake 30 of the first wind turbine. This is done by dividing the axial distance 38 between the first wind turbine 14 and the second wind turbine 15 by the propagation speed of the wake, which propagation speed in one embodiment corresponds to the wind speed obtained in step 110.

(31) In step 170 a control signal is generated at time point 25 or before time point 25 for altering a pitch angle of a rotor blade of the second wind turbine 15 relative to the pitch angle of another rotor blade of the second wind turbine 15. The method ends in step 180.

(32) Steps 120, 130, 140, 150, 160 are an exemplary mode of predicting a future time point 25 at which there is an overlap between a wake of the first wind turbine 14 and an area swept by a rotor of the second wind turbine 15. Other embodiments of determining the wake 30 and of determining the future time point 25 are described in this specification.