ESTIMATING FREE-STREAM INFLOW AT A WIND TURBINE

Abstract

Provide is a method of estimating free-stream inflow at a downstream wind turbine of a wind park, the method including: selecting, from plural candidate wind turbines previously defined specifically for the downstream wind turbine, an upstream wind turbine based on a currently determined wind direction; using determination equipment of the selected upstream wind turbine to determine the free-stream inflow.

Claims

1. A method of estimating free-stream inflow at a downstream wind turbine of a wind park, the method comprising: selecting, from plural candidate wind turbines previously defined specifically for the downstream wind turbine, an upstream wind turbine based on a currently determined wind direction; and determining using determination equipment of the selected upstream wind turbine, the free-stream inflow by utilizing free-stream inflow at the upstream wind turbine and correcting it with a time delay calculated from the wind speed, wind direction and a distance between the upstream wind turbine and the downstream wind turbine.

2. The method according to claim 1, wherein the upstream wind turbine is selected such that it is not, according to the currently determined wind direction, in a wake region of any other wind turbine, wherein the currently determined wind direction is initially determined by the downstream wind turbine and after selecting the candidate wind turbine is determined by the selected candidate wind turbine.

3. The method according to claim 1, wherein to each of the candidate wind turbines a wind direction angle range is associated such that the downstream turbine is arranged downstream of the candidate wind turbine within a cone corresponding to the wind direction angle range, wherein a candidate is selected for which at least a first criterion is satisfied, comprising: the currently determined wind direction is within the wind direction angle range associated with the selected candidate wind turbine.

4. The method according to claim 3, wherein that candidate wind turbine is selected, if further at least a second criterion is satisfied, comprising: the selected candidate wind turbine is closer to the downstream wind turbine than all other candidate wind turbines satisfying the first criterion.

5. The method according to claim 1, wherein a union of all wind direction angle ranges cover 360°, wherein at least two wind direction angle ranges overlap, in particular having an overlap of 5° to 20°.

6. The method according to claim 1, further comprising: determining a wind direction by the selected candidate wind turbine; checking the first criterion now applied to the wind direction determined by the selected wind turbine as the currently determined wind direction.

7. The method according to claim 6, comprising, if the first criterion is not satisfied for the selected candidate wind turbine: selecting another upstream wind turbine from the plural candidate wind turbines for which the first criterion, and in particular also the second criterion, is satisfied.

8. The method according to claim 1, wherein the candidate wind turbines are peripheral wind turbines of the wind park, wherein the selected candidate wind turbine is located in an angle range in front of the downstream wind turbine along the wind direction.

9. The method according to claim 1, wherein upon a change of the currently determined wind direction another wind turbine is selected from the candidate wind turbines, such that at least the first criterion satisfied.

10. The method according to claim 1, wherein if the changed wind direction is within an overlap of two wind direction angle ranges, the previously selected candidate wind turbine is maintained as the selected candidate wind turbine.

11. The method according to claim 1, wherein for at least one of the candidate wind turbines a primary backup wind turbine is selected in case the provisionally selected candidate wind turbine does not provide a reliable free-stream inflow determination, the primary backup wind turbine satisfying less stringent criteria than the candidates to be selected, wherein to at least one candidate wind turbine the wind direction angle range and a backup wind direction angle range is associated.

12. The method of controlling a downstream wind turbine and/or an upstream wind turbine, the method comprising: performing a method according to claim 1; and controlling, the rotor speed and/or blade pitch of the downstream wind turbine and/or the upstream wind turbine based on the free-stream inflow.

13. The method according to claim 12, wherein controlling the downstream and/or the upstream wind turbine includes: predicting, using a wake model, a wind condition at the downstream wind turbine based on the free-stream flow and operation conditions of all other wind turbines in front of the downstream wind turbine; and controlling the downstream and/or the upstream wind turbine based on the predicted wind condition at the downstream wind turbine.

14. An arrangement for estimating free-stream inflow at a downstream wind turbine of a wind park, the arrangement comprising: a selection module adapted to select, from plural candidate wind turbines previously defined specifically for the downstream wind turbine, an upstream wind turbine based on a currently determined wind direction; and determination equipment of the selected upstream wind turbine adapted to determine the free-stream inflow.

15. A wind park, comprising: plural wind turbines; and an arrangement according to claim 14 communicatively connected with the wind turbines, in particular adapted to perform a method for each non-peripheral wind turbine of the wind park as the downstream wind turbine.

Description

BRIEF DESCRIPTION

[0047] Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

[0048] FIG. 1 schematically illustrates a wind park according to an embodiment of the present invention;

[0049] FIG. 2 schematically illustrates a wind park according to an embodiment of the present invention;

[0050] FIG. 3, schematically illustrate candidate wind turbines and primary and secondary back-up candidate wind turbines as defined according to an embodiment of the present invention;

[0051] FIG. 4 schematically illustrate candidate wind turbines and primary and secondary back-up candidate wind turbines as defined according to an embodiment of the present invention;

[0052] FIG. 5 schematically illustrate candidate wind turbines and primary and secondary back-up candidate wind turbines as defined according to an embodiment of the present invention; and

[0053] FIG. 6 illustrates exemplary wind conditions around a wind turbine.

DETAILED DESCRIPTION

[0054] The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit.

[0055] The wind park 100 according to an embodiment of the present invention illustrated schematically in FIG. 1 comprises plural wind turbines 101a, 101b, 101c, 101d, 101e and 101f as well as an arrangement 103 for estimating free-stream inflow at a downstream wind turbine according to an embodiment of the present invention. Each wind turbine 101a, 101b, 101c, 101d, 101e and 101f has each respective rotor blades e.g. 102a, 102b rotating in a disk plane upon impact of wind.

[0056] The arrangement 103 is communicatively connected to all wind turbines capable of (bi)directional communication in particular receiving from all wind turbines operational and measurement signals 105 and supplying to the wind turbines control signals 107.

[0057] Thereby, the arrangement 103 is configured for carrying out a method of estimating free-stream inflow at a downstream wind turbine of a wind park according to an embodiment of the present invention. Therefore, the arrangement 103 has, for each downstream wind turbine 101b, previously defined plural candidate wind turbines (from which not all are illustrated in FIG. 1) shown as 101a, 101c, 101d.

[0058] Based on a currently determined wind direction 109, from the candidate wind turbines, an upstream wind turbine 101a is selected for the downstream wind turbine 101b, in order to use determination equipment 111a of the selected upstream wind turbine 101a to determine the free-stream inflow as would be experienced by the considered downstream wind turbine 101b if no other wind turbine is along the wind direction 109 in front of the considered downstream wind turbine 101b.

[0059] Thereby, the wind turbines 101a, 101c and 101d represent the candidate wind turbines which have previously been defined for the considered downstream wind turbine 101b. As can be appreciated from FIG. 1, the selected candidate wind turbine 101a is, according to the currently determined wind direction 109, not in a wake region of any other wind turbine of the wind park 100. The currently determined wind direction 109 may have been determined by determination equipment 111b of the considered downstream wind turbine 101b. To the selected wind turbine 101a as well as to all other candidate wind turbines 101a, 101c and 101d previously defined for the considered downstream wind turbine 101b, a corresponding wind direction angle range 113a, 113c and 113d is associated which may have an extension of for example 10 to 45°. The extension is indicated with Δλ. The extension Δχ is in fact in the horizontal direction (in a plan view as in FIG. 2 to 5), shown in FIG. 1 only for illustrative reasons as if on the vertical direction. The extension Δχ may equal or different for different wind turbines, e.g. smaller at higher density of turbines. Each wind turbine comprises rotor blades which rotate around a not illustrated rotation axis and which are mounted at a hub of the rotation axis.

[0060] The candidate wind turbines 101a, 101c, 101d are all (in particular a subset of) peripheral wind turbines forming a boundary of the wind park 100 and substantially surrounding all other wind turbines of the wind park 100. The considered downstream wind turbine 101b is located downstream of the selected candidate wind turbine 101a within a cone 115a whose top is located at the hub of the selected candidate wind turbine 101a and which is oriented to have a central axis 117 to be collinear with the central axis of the wind direction angle range 113a.

[0061] As can be taken from FIG. 1, the currently determined wind direction 109 is within the wind direction angle range 113a of the selected candidate wind turbine 101a but not within the wind direction angle ranges 113c, 113d of the other candidate wind turbines 101c, 101d. Furthermore, the selected wind turbine 101a is relatively close to the considered downstream wind turbine 101b such that no other candidate (or peripheral) wind turbine is in front of the considered downstream wind turbine 101b and closer to the considered downstream wind turbine 101b.

[0062] If the currently determined wind direction 109 would be different from the direction as indicated in FIG. 1, another one of the candidate wind turbines other than the wind turbine 101a may be selected in order to provide free-stream inflow information with respect to the downstream wind turbine 101b.

[0063] The operational and measurement signals 105 may therefore include information regarding free-stream inflow as determined by the determination equipment 111a of the selected candidate wind turbine 101a. Based on the free-stream inflow information received by the arrangement 103, the arrangement 103 computes control signals 107 to control the upstream wind turbine 101a and/or the downstream wind turbine 101b, in particular regarding yawing the wind turbine which involves rotating the wind turbines (in particular nacelles) around a vertical axis 119a, 119b, respectively, and/or changing pitch angle and/or changing power output and/or changing or controlling rotational speed and the like. For the selection, the arrangement 103 comprises a selection module 104.

[0064] Purpose of functionality according to embodiments of the present invention may be to ensure that the wind condition measurements that are used at each turbine to determine the optimal control settings for wake, are measured at a turbine in the wind farm which may comprise one or more of the following criteria: [0065] 1. It is not in the wake of another wind turbine at the time of measurement, i.e. it is a measurement of the free-stream flow. [0066] 2. It is relatively close to the yaw-offset-controlled turbine (i.e. the downstream turbine 101b), in a cross-flow and along the flow direction, such that the measured free-stream flow is representative of what the yaw controlled turbine (wind turbine 101b) would have as inflow if it would not be wake, impacted.

[0067] For this purpose, based on the layout of the wind farm, for each yaw-offset-controlled wind turbine in the wind farm, a group of possible candidate turbines can be defined, from which to receive measurements of the free-stream wind conditions. Each of the turbines in this “reference group” (also referred to as candidates) may have a certain wind direction range, such as ranges 113a,c,d, . . . , (or sector) in which its wind direction and speed measurements can be used as a valid free-stream flow measurement that may fulfil the above requirements.

[0068] FIG. 2 schematically illustrates an example of the definition of the reference group (also referred to as candidates) and its sectors (wind direction angle ranges). In FIG. 2, the reference group of wind turbines (or candidate wind turbines) comprise the wind turbines A03, A02, A01, B01, C01, F02, F07 and B07 as indicated which are candidate wind turbines of the considered downstream wind turbine B02. The considered downstream wind turbine B02 of the wind park 200 illustrated in FIG. 2 will utilize the measurements of the free-stream wind direction, speed and turbulence from a candidate wind turbine which is selected based on the currently determined wind direction. Thereby, the sectors or wind direction angle ranges are indicated with reference sign 113. Whenever the determined wind direction is within a wind direction angle range of a particular candidate wind turbine, that candidate wind turbine is selected and a wind measurement or wind determination derived from this selected wind turbine is used for deriving the free-stream inflow for the considered downstream wind turbine B02. As has been mentioned, each of the candidate wind turbines has a certain wind direction range (sector) 113, indicated in which its wind direction and speed measurements can be used by a yaw-controlled turbine B02 as the free-stream flow measurements. When the reference turbine (i.e. the selected candidate turbine) needs to be changed (for example the wind direction is out of the sector of the reference turbine), the free-stream flow measurement may be switched to being taken at a new reference turbine by checking which sector corresponds to the wind direction.

[0069] In order to prevent jumps in the free-stream flow measurement signals, a rate-off-change limitation of the measurement signals may be applied.

[0070] Each of the wind direction angle ranges (i.e. each sector) 113 may be chosen such that wake interference may be avoided. There may a margin (in this case of at least 30°) between a sector boundary and the alignment angle with another turbine. The combination of the sectors may cover the full 360° range of wind directions, as illustrated with the encirclement 121 of the turbine B02.

[0071] By using a small overlap of the sectors, some hysteresis may be used when switching between reference turbines, in which the turbine will not change reference turbine as long as it is within the sector of that reference turbine. This may prevent rapid back and forth switching between reference turbines.

[0072] For some robustness to measurement down-time on some turbines, the order of the reference group could be defined in such a way that there are one or more “back-up” turbines used in case a turbine is down or for some other reason not providing a valid measurement. An example is illustrated in the following FIGS. 3, 4 and 5, each showing a respective wind park 300, 400 and 500, respectively.

[0073] FIG. 3 illustrates the situation of FIG. 2 showing the candidate wind turbines A03, A02, A01, B01, F02, F07 and B07 of the considered downstream wind turbine B02. In FIG. 4 it is illustrated that the wind turbine A02 may serve as a back-up wind turbine for the wind turbine A03 in case the wind turbine A03 cannot provide a reliable wind determination. In this back-up case the wind turbine A02 also provides the wind determination data if the currently determined wind direction is within the back-up wind direction angle range 114 which had previously been associated as the wind direction angle range 113 for the wind turbine A03. Thus, the neighbouring turbine of the previously selected turbine could be used as a back-up turbine, using the same sector as the original turbine. When including back-up turbines and second back-up turbines (illustrated for example in FIG. 5) (used in case the measurement of the back-up turbine is not available either), the method may further be improved regarding robustness. In the FIG. 5, the wind turbine A03 may serve as a secondary back-up wind turbine for the wind turbine A02 and the wind turbine A01 may serve as a secondary back-up of the wind turbine B01, both having secondary wind direction ranges 116 associated.

[0074] FIG. 6 illustrates wind conditions around a wind turbine having rotor disk 120 with rotor blades. Free-stream wind 109 impacts on the rotor disk 120 generating a wake region 122 downstream the rotor disk 120. The disk exerts a force 130 on the wind flow. The wind velocity 124 is at the free-stream level 126 upstream the rotor disk 120, decreases in a region downstream to a minimum and further downstream increases to almost reach the free-stream level 126. Within a space 128 mixing between turbulent air and free-stream wind occurs.

[0075] Embodiments of the present invention may provide several advantages: [0076] very little communication between turbines and signal processing needed to establish reference turbine [0077] robustness to measurement down time [0078] smoothness of measurement signals

[0079] Each of these may contribute to smoothing and availability of measurement signals. Smooth and reliable measurements may be needed for robust wake mitigation control in the wind farm.

[0080] According to embodiments of the present invention, locally predefined activation sectors (i.e. wind direction angle ranges) are defined rather than an iterative method is applied, providing e.g. [0081] robustness to measurement down time by using back-up reference turbine [0082] rate-of-change limitation on signal and preventing unnecessary back-up-and-forth switching between reference turbines.

[0083] The resulting robust wake mitigation control in the wind farm may improve energy production and/or reduction of mechanical or electrical loads on the wind turbines.

[0084] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0085] For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module.