Method of wake steering wind turbine

Abstract

A method of wake steering a first wind turbine. The second wind turbine is positioned downstream of the first wind turbine such that the second wind turbine may be affected by a wake of the first wind turbine. A yaw offset signal is applied to a yaw control system of the first wind turbine, the yaw offset signal causing the yaw control system to generate a yaw offset between a rotor of the first wind turbine and a wind direction, the yaw offset diverting the wake away from the second wind turbine. The yaw offset controller is operated to vary the yaw offset signal in response to a change in the wind direction. In response to detecting that the wind direction crosses a full wake wind direction, a timing system is activated and operation of a yaw offset controller is maintained such that a yaw offset signal does not change in response to a change in the wind direction. Operation of the yaw offset controller is resumed in response to either timeout of the timekeeping system or detecting that the wind direction re-crosses the full wake wind direction prior to timeout of the timekeeping system.

Description

Methods for wake steering of wind turbines Technical Field

[0001] The present invention relates to a method for wake steering of a wind turbine, a yaw offset controller configured to control the wind turbine by such method, and a wind turbine including such a yaw offset controller. Background Technology

[0002] In a wind farm comprising multiple wind turbines, a second wind turbine among the multiple wind turbines may be affected by the wake region caused by a first wind turbine located upstream of the second wind turbine.

[0003] US2021 / 207580 A1 discloses a known method for controlling a first wind turbine among a plurality of wind turbines in a wind farm. The method includes: determining a current yaw state; determining wind conditions indicating a potential wake-induced level at least caused by the first wind turbine at a second wind turbine; and defining a rotor yaw offset angle setpoint for the first wind turbine based on the current yaw state, the wind conditions, and at least one yaw angle hysteresis switching threshold. The rotor yaw offset angle setpoint follows a hysteresis, thereby avoiding immediate and continuous switching between a range of positive and negative yaw offset angles.

[0004] It would be beneficial to provide further ways to control wind turbines in a wind farm that are affected by the wakes of other wind turbines in the wind farm. Summary of the Invention

[0005] A first aspect of the present invention provides a method for wake steering of a first wind turbine, wherein a second wind turbine is positioned downstream of the first wind turbine such that the second wind turbine is affected by the wake of the first wind turbine, the method comprising: applying a yaw offset signal to a yaw control system of the first wind turbine, the yaw offset signal causing the yaw control system to generate a yaw offset between the rotor of the first wind turbine and the wind direction, the yaw offset steering the wake away from the second wind turbine; operating a yaw offset controller to change the yaw offset signal in response to a change in wind direction; detecting that the wind direction crosses the full wake direction, wherein the full wake direction is parallel to the heading between the first and second wind turbines; in response to detecting that the wind direction crosses the full wake direction: activating a timing system and maintaining operation of the yaw offset controller such that the yaw offset signal does not change in response to a change in wind direction; and resuming operation of the yaw offset controller in response to a timeout of the timing system.

[0006] Optionally, the timing system includes a timer; the timing system is started by activating the timer, and the timeout of the timing system is caused by the timer reaching a timeout threshold.

[0007] Optionally, the method further includes changing the timeout threshold after the timer has been started.

[0008] Optionally, in response to the wind direction deviating from the full wake direction, the timeout threshold decreases to a reduced timeout threshold; and the timeout of the timing system is caused by the timer reaching the reduced timeout threshold. The timeout threshold is inversely correlated with the wind direction, and therefore decreases in response to the wind direction deviating from the full wake direction.

[0009] Optionally, the timeout threshold varies according to a parameter related to wind variability, such that for a difference between the wind direction and the entire wake direction, the timeout threshold increases with the increase of the parameter related to wind variability, up to at least an upper limit. The parameter related to wind variability can be based on turbulence intensity parameters, standard deviation of wind speed, standard deviation of wind direction, or changes in blade load sensor signals.

[0010] Optionally, the timing system includes a first timer having a first timeout threshold and a second timer having a second timeout threshold shorter than the first timeout threshold, and the method includes: in response to detecting that the wind direction crosses the full wake wind direction: starting the operation of the first timer and maintaining the operation of the yaw offset controller; in response to detecting that the wind direction crosses the threshold before the first timer reaches the first timeout threshold, starting the second timer; and in response to the first timer reaching the first timeout threshold or the second timer reaching the second timeout threshold, resuming the operation of the yaw offset controller.

[0011] A second aspect of the invention provides a method for wake steering of a first wind turbine, wherein a second wind turbine is positioned downstream of the first wind turbine such that the second wind turbine is affected by the wake of the first wind turbine, the method comprising: applying a yaw offset signal to a yaw control system of the first wind turbine, the yaw offset signal causing the yaw control system to generate a yaw offset between the rotor of the first wind turbine and the wind direction, the yaw offset steering the wake away from the second wind turbine; operating a yaw offset controller to change the yaw offset signal in response to a change in wind direction; detecting that the wind direction crosses the full wake direction, wherein the full wake direction is parallel to the heading between the first and second wind turbines; in response to detecting that the wind direction crosses the full wake direction: activating a timing system and maintaining the operation of the yaw offset controller such that the yaw offset signal does not change in response to a change in wind direction; and in response to detecting that the wind direction crosses the full wake direction again before the timing system expires, resuming the operation of the yaw offset controller.

[0012] Optionally, detecting the wind direction crossing the full wake wind direction includes detecting the sign change of the yaw offset signal.

[0013] The following comments apply to the first or second aspect of the present invention.

[0014] Optionally, resuming the operation of the yaw offset controller causes the yaw offset controller to change the sign of the yaw offset signal.

[0015] Optionally, the yaw offset signal is changed by the yaw offset controller based on a transfer function.

[0016] Optionally, the transfer function includes: a negative offset band, in which the yaw offset signal is negative and decreases to a minimum value; a positive offset band, in which the yaw offset signal is positive and increases to a maximum value; and a transition or shift between each offset band, in which the sign of the yaw offset signal changes.

[0017] Another aspect of the invention provides a yaw offset controller configured to control a first wind turbine by means of a method according to the first or second aspect.

[0018] Another aspect of the present invention provides a wind turbine, comprising: a rotor; a yaw control system; and a yaw offset controller according to the foregoing aspect.

[0019] The yaw offset controller can be a control unit or control module, such as a unit or module included in a computer program. Such a unit or module can be implemented to receive input signals, determine yaw offset signals, perform the method according to the first or second aspect of the invention, and output / apply yaw offset signals to the yaw control system of the wind turbine.

[0020] In one aspect, the present invention also relates to a computer program including instructions that, when executed by a computer, cause the computer to perform the method according to the first or second aspect.

[0021] A computer program can be a computer program product and can take the form of a computer-readable storage medium or be downloadable from a communication network. A computer program includes instructions, such as executable instructions in the form of software code, which, when executed by a computer, cause the wind turbine's control system to perform the method according to the first or second aspect.

[0022] When loaded onto the data processing system of the control system, the computer program can cause the yaw offset controller to execute instructions. Typically, the yaw offset controller can be a collection of units or functional units, including one or more processors, input / output interfaces, and memory capable of storing instructions that can be executed by the processor.

[0023] A yaw offset controller can be implemented as part of the wind turbine's control system. Such a control system can be a local control system located at the wind turbine, or a centralized control system centrally located and communicating with the wind turbine's control system (e.g., with the yaw control system and / or yaw offset controller) via a communication network. The central control system can be located within one wind turbine and communicate with another wind turbine via a communication network. Alternatively, the central control system can be located within a central control system (e.g., a SCADA control system or a power plant control system) and communicate with the wind turbine via a communication network. The yaw offset controller can also be part of a distributed control system distributed among controller elements of the wind turbine's control system, potentially including elements located locally on the wind turbine and central controller elements. Attached Figure Description

[0024] Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a wind turbine; Figure 2A shows a control system for the wind turbine; Figure 2B shows the various components of the yaw offset controller; Figure 3 shows the transfer function; Figure 4 shows the operating state at the first point of the transfer function; Figure 5 indicates the second point of the transfer function; Figure 6 shows the operating state at the second point; Figure 7 indicates the third point of the transfer function; Figure 8 shows the operating state at the third point; Figure 9 indicates the fourth point of the transfer function; Figure 10 shows the operating state at the fourth point; Figure 11 shows the wind direction and yaw offset signals for the long timer; Figure 12 shows the wind direction and yaw offset signals for the short timer; Figure 13 shows the yaw offset controller operating to change the yaw offset signal in response to changes in wind direction. The components; Figure 14 shows the yaw offset controller of Figure 13 in a held state; Figure 15 is a first example of the timing system of the yaw offset controller of Figure 13; Figure 16 shows the wind direction and yaw offset signals when the wind direction oscillates around the full wake direction; Figure 17 shows an alternative timing system; Figure 18 shows another alternative timing system; Figure 19 shows yet another alternative timing system; Figure 20 is a graph showing the timer output changing over time; Figure 21 is a graph showing the timer output when a recrossing is detected; Figure 22 is a graph showing how the timer output changes in an alternative embodiment; and Figure 23 is a graph showing how the timer output changes in yet another alternative embodiment; and Figure 24 shows an embodiment where the timeout threshold changes according to parameters related to wind variability. Detailed Implementation

[0025] Figure 1 shows a schematic perspective view of a first wind turbine 1. The wind turbine 1 includes a tower 2 and a rotor-nacelle assembly (RNA) located on top of the tower 2. The RNA includes a nacelle 3 and a rotor 4, the rotor 4 being operatively coupled to a generator housed within the nacelle 3. In addition to the generator, the nacelle 3 also houses various components required for converting wind energy into electrical energy, as well as various components required for operating and controlling the wind turbine 1.

[0026] The rotor 4 includes a central hub and multiple blades 5 projecting outward from the central hub. In the wind turbine 1 shown, the rotor 4 includes three blades 5, but the number can vary.

[0027] Figure 2A schematically illustrates one embodiment of the control system 20 and the components of the wind turbine 1. The rotor is mechanically connected to the generator 7 via a gearbox 9 (there is no gearbox in a direct-drive system). The electrical energy generated by the generator 7 is injected into the power grid 24 via an electric converter 25. The generator 7 and converter 25 can be based on a full-size converter (FSC) architecture or a doubly fed induction generator (DFIG) architecture, but other types may also be used.

[0028] The control system 20 includes multiple elements, including at least one main controller 10 with a processor and memory, enabling the processor to perform computational tasks based on instructions stored in the memory. Typically, the main controller 10 ensures that the wind turbine produces the requested power output level during operation. This is achieved by adjusting the blade pitch angles and / or the power extraction of the converter 25. For this purpose, the control system 20 includes: a pitch system including a pitch controller 27 using a pitch reference signal 28; and a power system including a power controller 29 using a power reference signal 26. The rotor blades 5 can be pitched via a pitch mechanism. The rotor includes an independent pitch system capable of independently pitching the rotor blades 5, and may include a common pitch system that simultaneously adjusts all pitch angles on all rotor blades. The control system 20 or its elements can be connected to a power plant controller (not shown) or other control systems to receive externally provided instructions.

[0029] The wind turbine 1 includes a yaw control system 31 configured to rotate the rotors 3 and 4 about a vertical yaw axis to control the yaw angle of the rotor 4. The yaw control system 31 includes various components, including a yaw motor. The control system 20 includes a yaw offset controller 30 configured to generate a yaw offset signal 32 input to the yaw control system 31.

[0030] The yaw offset controller 30 is configured to control the wind turbine 1 by changing the yaw offset signal 32 in response to changes in wind direction. The yaw offset signal 32 can be changed based on a transfer function stored in the memory of the yaw offset controller 30. One possible transfer function is explained in more detail below.

[0031] Yaw offset controller 30 is shown as part of control system 20. In an embodiment, the yaw offset controller may alternatively be located in a remote controller and communicatively connected to the wind turbine's control system. Such a remote controller may be a central controller tasked with determining the yaw offset setpoint of the wind turbines in the wind farm. The central controller may be implemented, for example, in a SCADA system communicatively connected to the wind turbines in the wind farm. Alternatively, the central controller may be implemented in a power plant control system arranged to control the wind turbines in the wind farm. The central controller may form part of a distributed system comprising: a central element, such as a computing element for calculating the yaw offset setpoint; and local elements, such as local controller elements arranged to receive the yaw offset setpoint and control the yaw of the wind turbines.

[0032] Figure 2B provides a schematic overview of the components of the yaw offset controller 30 and a portion of the yaw control system 31. The yaw offset controller 30 includes a sensor and processing unit 80 and a yaw offset control module 81; the yaw control system 31 includes an upwind yaw control module 82.

[0033] Sensor 80 receives wind force and calibrated cabin heading as input; and outputs wind direction and wind speed to modules 81 and 82.

[0034] The yaw offset control module 81 outputs a yaw offset signal 32 to the windward yaw control module 82. The yaw offset signal 32 is based on the wind direction and the aforementioned transfer function, which can be stored by the yaw offset module 81 in the form of a lookup table or in any other manner. Alternatively or supplementing to the lookup table, the yaw offset signal can be calculated using a computer model arranged to simulate the wake effect and the interactions between turbines in a wind farm.

[0035] The wind direction used by the yaw offset control module 81 can be the instantaneous wind direction or a low-pass filtered signal based on the wind direction.

[0036] The windward yaw control module 82 outputs a yaw motor activation signal to the yaw motor based on the yaw offset signal 32 (which gives the target yaw offset) and the wind direction and wind speed.

[0037] Figure 3 shows an example of a transfer function that can be used by the yaw offset controller 30.

[0038] The transfer function can be stored in the memory of the yaw offset controller 30 of the wind turbine 1. In other embodiments of the invention, the transfer function can be stored at a location remote from the wind turbine 1, such as at a central location.

[0039] The transfer function can be stored in the form of a lookup table. In other embodiments of the invention, the transfer function can be obtained through online optimization or the like.

[0040] As shown in Figure 4, the second wind turbine 40 is positioned downstream of the first wind turbine 1, such that the second wind turbine 40 may be affected by the wake 41 of the first wind turbine 1. The direction between the turbines 1 and 40 is indicated by heading 14. When the wind 13 is parallel to the heading 14 between the first and second wind turbines 1 and 40, this wind direction is referred to as the full wake wind direction.

[0041] Figure 4 shows the wind direction and yaw angle of wind turbines 1 and 40 at the first point 1a of the transfer function in Figure 3. For this wind direction +α, the yaw offset signal 32 is set to zero, so the rotor axis 12 is parallel to the wind 13. Here, the wind direction +α is defined as the angle of the wind 13 relative to the heading 14.

[0042] Since the wind direction + α is a relatively large angle, under the operating conditions shown in Figure 4, the wake 41 will not significantly affect the second wind turbine 40.

[0043] As the wind direction decreases from +α, thus approaching the full wake direction, the transfer function causes a gradually increasing yaw offset to be applied until it reaches its maximum value at point 2a shown in Figures 5 and 6. For this wind direction +β, the yaw offset signal 32 is at its maximum value +θ, therefore, as shown in Figure 6, the rotor axis forms an angle of +θ with the wind. Due to the wake steering caused by the yaw offset, the wake 41 does not significantly affect the second wind turbine 40 under the operating conditions shown in Figure 6.

[0044] As the wind direction decreases from +β, the transfer function is flat until point 3a, as shown in Figures 7 and 8, where wind 13 is in the full wake direction.

[0045] For this part of the transfer function, the yaw offset signal 32 remains at its maximum value of +θ, so as shown in Figure 8, the rotor axis remains at an angle of +θ to the wind. At point 3a, the wake 41 may affect the second wind turbine 40, as shown in Figure 8.

[0046] Next, the transfer function undergoes a step transition, where it jumps from point 3a to point 4a as shown in Figure 9. The yaw offset signal 32 jumps to a minimum angle of -θ, so that, as shown in Figure 10, the rotor axis forms an angle of -θ with the wind direction. At point 4a, the wake 41 may affect the second wind turbine 40, as shown in Figure 10.

[0047] The remainder of the transfer function in Figure 3 will not be described because the transfer function is symmetric.

[0048] Figure 11 is a graph, where line 50 indicates wind direction based on the left-hand scale. Wind direction is given in degrees, with the direction of the entire wake specified as the wind direction at zero degrees.

[0049] Lines 51 and 52 illustrate two different yaw offset signals based on the right-hand scale. A yaw offset of +1 corresponds to the angle +θ in the transfer function of Figure 3, and a yaw offset of -1 corresponds to the angle -θ in the transfer function of Figure 3.

[0050] Figure 11 can show time periods of several minutes (e.g., 10 minutes or longer). Therefore, as an example, time 1 on the time scale of Figure 1 could be 10 minutes. As a non-limiting example, the following discussion will assume that time 1 on the time axis of Figure 11 corresponds to 10 minutes.

[0051] In Figure 11, wind direction 50 crosses the entire wake direction in approximately 3 minutes. The transfer function in Figure 3 exhibits a step transition, where it jumps from point 3a to point 4a by an angle of 2θ, which can be on the order of 40 degrees. The vertical portion of the dashed line 51 in Figure 11 illustrates the immediate sign switching of the yaw offset signal in response to wind direction 50 crossing the entire wake direction.

[0052] The large size of this step transition can cause wear on the yaw control system 31, especially if the wind direction changes rapidly around the full wake direction. Figure 13 shows an example of how to modify the yaw offset controller 30 to address this problem.

[0053] The yaw offset signal 32 is input to a zero-crossing (or zero-crossing) detector 60, which detects a step transition (in either direction) in the yaw offset signal between points 3a and 4a. The output of the zero-crossing detector 60 is input to a timing system 61. The Boolean output of the timing system 61 is input to a switch 62, and the output of the switch 62 is the modified yaw offset signal input to the yaw control system 31.

[0054] During the 0 to 3 minute time period in Figure 11, the Boolean output of timing system 61 is zero, and switch 62 is in the position shown in Figure 13. Therefore, the modified yaw offset signal is the same as the unmodified yaw offset signal 32.

[0055] When the zero-crossing detector 60 detects a step transition (caused by a change in wind direction to negative) at 3 minutes, the Boolean output of the timing system 61 becomes non-zero, and switch 62 switches to the position shown in Figure 14. Therefore, the modified yaw offset signal is based on the 1 / z input. As a result, the modified yaw offset signal does not follow the step transition of the yaw offset signal 32, but maintains its current value, as shown by the dashed line 52 in Figure 11. Therefore, in response to the detection of wind direction crossing the full wake wind direction, the operation of the yaw offset controller 30 is maintained, so that the modified yaw offset signal 52 does not change in response to changes in wind direction.

[0056] The timing system 61 is also activated in response to the detection of wind direction crossing the entire wake direction. In the example of Figure 11, the timing system 61 includes a long timer that is activated in response to the detection of wind direction crossing the entire wake direction and expires after 4 minutes.

[0057] During the hold time of timing system 61 (i.e., before timing system 61 times out), the yaw offset signal before the switch is retained. The timeout of timing system 61 causes its Boolean output to return to zero, thus switch 62 returns to the position shown in Figure 13. Therefore, in response to the 4-minute timeout of timing system 61, the operation of yaw offset controller 30 is resumed. In the example of Figure 11, resuming the operation of yaw offset controller 30 causes the yaw offset controller to change the sign of the yaw offset signal. That is, the yaw offset signal 52 changes sign after approximately 7 minutes, because the wind direction is still negative at this time.

[0058] In this example, the yaw offset signal 52 jumps by 2 (i.e., the angle changes by 2θ) because during the recovery operation, the wind direction is between -β and zero, and therefore in the flat part of the transfer function in Figure 3.

[0059] If the wind direction is less than -β when resuming operation of the yaw offset controller, the yaw offset signal 52 will change sign, but the jump will be less than 2, depending on the point in the transfer function.

[0060] If the wind direction is positive when the yaw offset controller is restored to operation, the yaw offset signal 52 will not change sign.

[0061] Figure 12 shows the modified yaw offset control signal 53 for a long timer (with a 4-minute timeout) and the modified yaw offset control signal 54 for a short timer (with a 3-minute timeout).

[0062] Figure 15 shows an example of a timing system 61, which includes a latch 70 triggered by the output of a zero-crossing detector 60. The output of latch 70 is a Boolean output of timing system 61.

[0063] When the Boolean output of latch 70 becomes true, timer 71 starts counting. The output of timer 71 is subtracted from the timeout threshold 73 by subtractor 72. When the output of subtractor 72 becomes negative, latch 70 is released, and the Boolean output of timing system 61 toggles to restore the operation of the yaw offset controller by operating switch 62. The release of latch 70 also resets timing system 61 by resetting timer 71 to zero.

[0064] In a simple example, the timeout threshold 73 can be fixed, for example, 3 minutes or 4 minutes in the example of Figure 12.

[0065] In the example of Figure 15, the timeout threshold 73 varies based on a function 74 of the angle between the wind direction and the full wake direction. In this example, the function 74 has a linear slope 75 that varies continuously between a high value 76 (when the wind direction is close to the full wake direction) and a low value 77 (when the wind direction is farther from the full wake direction). Note that the timeout threshold is inversely correlated with the wind direction and therefore decreases in response to the wind direction deviating from the full wake direction.

[0066] After timer 71 starts counting, function 74 can respond to wind direction deviating from the full wake direction, causing timeout threshold 73 to decrease to a reduced timeout threshold. For example, timeout threshold 73 can decrease from a high value 76 to a low value 77, or decrease to an intermediate value at some point on the linear ramp 75. Timeout of timing system 61 is caused by the timer reaching a reduced timeout threshold, which can continuously vary based on function 74.

[0067] Figure 16 shows a time period during which the wind direction crosses the entire wake direction four times, as indicated by solid line 50. In a conventional system, this would result in rapid back-and-forth switching of the yaw offset signal, as shown by dashed line 51 in Figure 15.

[0068] In the example of Figure 16, although the wind direction crosses the entire wake direction four times, the yaw offset signal output to the yaw control system 31 remains constant, as shown by the dashed line 52.

[0069] The first zero crossing occurs at approximately 1.5 minutes, causing timer 71 to start counting. The wind direction crosses the full wake again at approximately 3 minutes, at which point timing system 61 has not yet timed out.

[0070] Figure 17 illustrates an alternative timing system 61, which includes a recrossing (or backcrossing) detection signal 78 that releases latch 70 in response to the wind direction recrossing the full wake wind direction approximately 3 minutes before the timing system 61 times out. The release of latch 70 also resets the timing system 61 by resetting timer 71 to zero. At the next zero crossing in Figure 16 (exactly 6 minutes prior), latch 70 is triggered again.

[0071] The recrossing detection signal 78 can be generated by detecting changes in the sign of the yaw offset signal, or by detecting the wind direction crossing the entire wake in some other way.

[0072] Figure 18 illustrates an alternative timing system 61 that achieves a similar effect to the alternative timing system in Figure 17, but without the recrossing detection signal 78. In this case, function 74 is modified to include a step transition 79 (at zero angle), where the timeout threshold 73 switches from positive to negative. This causes timing system 61 to time out immediately, releasing latch 70 and resetting the timing system by resetting timer 71 to zero. Step transition 79 has a similar effect to the recrossing detection signal 78, releasing latch 70 in response to the wind direction recrossing the full wake wind direction before timing system 61 times out. At the next zero crossing in Figure 16 (exactly 6 minutes prior), latch 70 is triggered again.

[0073] The function 74 in Figures 17 and 18 can be piecewise continuous and piecewise differentiable.

[0074] Figure 19 illustrates yet another alternative timing system 61. In this case, two timers are provided: a first timer 71a with a first (long) timeout threshold (e.g., 5-10 minutes) and a second timer 71b with a second (short) timeout threshold (e.g., 1-2 minutes) shorter than the first timeout threshold.

[0075] When the Boolean output of latch 70 becomes true, the first timer 71a starts counting. The output of the first timer 71a is subtracted from the long timeout threshold by subtractor 72a.

[0076] When the wind direction deviates from the full wake direction to the threshold, the second timer 71b starts counting before the first timer 71a reaches the first timeout threshold. The output of the second timer 71b is subtracted from the short timeout threshold by the subtractor 72b.

[0077] When the output of either subtractor 72a or 72b becomes negative, a timeout occurs in timing system 61. This causes latch 70 to be released, and the Boolean output of timing system 61 toggles to resume operation of the yaw offset controller via operating switch 62. The release of latch 70 also resets timing system 61 by resetting timers 71a and 71b to zero.

[0078] Therefore, in Figure 19, the operation of the yaw offset controller is resumed in response to the first timer 71a reaching the first timeout threshold or the second timer 71b reaching the second timeout threshold.

[0079] In Figure 19, the timing system 61 has two timers 71a and 71b with different timeout thresholds. The number of timers is not limited to two; therefore, in other embodiments, the timing system 61 may have three or more timers, each with a different timeout threshold.

[0080] Figure 20 is a graph where line 100 shows the change of the output of timer 71 over time in any of the preceding embodiments. The timer output counts from zero upwards until a timeout threshold 101 is reached. When the timeout threshold 101 is reached, the operation of the yaw offset controller is resumed, and the timer is reset.

[0081] Figure 21 is a graph showing the output of timer 71 when a recrossing is detected in the embodiment of Figure 17 or Figure 18. The timer output counts from zero upwards until a recrossing detection occurs at time T1, i.e., a recrossing of the wind direction across the full wake is detected. In response to the recrossing detection at time T1, timing system 61 is reset (as shown by vertical line 102), and operation of the yaw offset controller immediately resumes.

[0082] Figure 22 is a graph illustrating how the timer output changes in an alternative embodiment. The timer output counts upwards from zero until a recrossing detection occurs at time T1, i.e., a recrossing of the full wake is detected. In response to the recrossing detection at time T1, the timer starts counting downwards until it reaches zero at time T2, causing timing system 61 to time out. In response to the timeout of timing system 61 at time T2, the operation of the yaw offset controller is resumed. Note that in other examples, the timeout of timing system 61 is caused by the timer output reaching a threshold. In this example, the timeout of timing system 61 is caused by the timer output returning to zero at time T2.

[0083] Figure 23 is a graph illustrating how the timer output changes in yet another alternative embodiment. The timer output counts upwards from zero until a recrossing detection occurs at time T1, i.e., a recrossing of the full wake wind direction is detected. In response to the recrossing detection at time T1, the timer starts counting downwards until another recrossing detection occurs at time T2 before the timer reaches zero. After time T2, the timer output starts counting upwards again until a timeout threshold 101 is reached. When the timeout threshold 101 is reached at time T3, the operation of the yaw offset controller is resumed, and the timer is reset.

[0084] In various embodiments, the timeout threshold 73 can vary based on a function 74 of the angle between the wind direction and the full wake direction, for example, based on the difference (angle difference) between the wind direction and the full wake direction. In the illustrated embodiment, the function 74 has a linear slope 75 that varies continuously between a high value 76 (when the wind direction is close to the full wake direction) and a low value 77 (when the wind direction is farther from the full wake direction). When above the minimum difference between the wind direction and the full wake direction, the timeout threshold is inversely correlated with the difference between the wind direction and the full wake direction, and therefore decreases in response to the wind direction deviating from the full wake direction.

[0085] Function 74 varies the timeout threshold as a function of wind direction, but in other embodiments of the invention, the timeout threshold may vary as a function of other parameters, such as parameters related to wind variability, hereinafter based on turbulence intensity parameters, standard deviation of wind speed, standard deviation of wind direction, or variations in blade load sensor signals.

[0086] Figure 24 illustrates one embodiment of function 74, where the timeout threshold is further varied according to a parameter related to wind variability, such that for the same difference between the wind direction and the full wake direction, the timeout threshold increases 79 as the parameter related to wind variability increases, at least up to an upper limit and at least up to a minimum threshold. The upper limit is shown by curve segments 76C, 75C, and 77.

[0087] For example, as the turbulence intensity TI increases, the piecewise linear threshold moves from the basic curves 75 and 76 to the maximum curves 75C and 76C, passing through one or more intermediate curves 75B and 76B. These curves have the same low value 77, but reach the low value along different piecewise linear curves.

[0088] Turbulence intensity can be defined in different ways, and is defined below as the standard deviation of wind speed divided by the average wind speed, the standard deviation of relative wind direction divided by the average wind speed, or the standard deviation of relative wind direction.

[0089] The timeout threshold can be set as a function of turbulence intensity and wind direction difference, such that for cases where the difference between the wind direction and the full wake direction is below a minimum of 78, constant timeout thresholds 76, 76B, and 76C are set, which increase by 79 as the turbulence intensity increases from a base timeout threshold 76 to a maximum timeout threshold 76C. For cases where the difference between the wind direction and the full wake direction is above a minimum of 78, the timeout threshold decreases linearly (piecewise) as the difference increases until a specific difference is reached, at which point the timeout threshold reaches a minimum of 77. In addition to decreasing as a function of the difference, the timeout threshold increases by 79 as the turbulence intensity increases from a base timeout threshold 76 to a maximum timeout threshold 76C. These curves can be implemented in different ways, for example, by setting a constant level in the region below the minimum difference between the wind direction and the full wake direction based on a function, and by setting the slope in the region above the minimum difference between the wind direction and the full wake direction based on various functions. Those skilled in the art can design further and different implementations that vary the timeout threshold according to various parameters.

[0090] Although the invention has been described above with reference to one or more preferred embodiments, it should be understood that various changes or modifications may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method for wake steering of a first wind turbine, wherein, The second wind turbine is positioned downstream of the first wind turbine, such that the second wind turbine can be affected by the wake of the first wind turbine. The method includes: applying a yaw offset signal to a yaw control system of the first wind turbine, the yaw offset signal causing the yaw control system to generate a yaw offset between the rotor of the first wind turbine and the wind direction, the yaw offset turning the wake away from the second wind turbine; operating a yaw offset controller to change the yaw offset signal in response to a change in wind direction; detecting that the wind direction crosses the full wake direction, wherein the full wake direction is parallel to the heading between the first and second wind turbines; in response to detecting that the wind direction crosses the full wake direction: activating a timing system and maintaining the operation of the yaw offset controller such that the yaw offset signal does not change in response to a change in wind direction; and resuming the operation of the yaw offset controller in response to either: a timeout of the timing system, or detection that the wind direction crosses the full wake direction again before the timeout of the timing system.

2. The method according to claim 1, wherein, In response to a timeout in the timing system, the operation of the yaw offset controller is resumed.

3. The method according to claim 2, wherein, The timing system includes a timer; the timing system is started by activating the timer, and the timeout of the timing system is caused by the timer reaching a timeout threshold.

4. The method of claim 3 further includes changing the timeout threshold after the timer has been started.

5. The method according to claim 4, wherein, In response to the wind direction deviating from the full wake direction, the timeout threshold is reduced to a reduced timeout threshold; and the timeout of the timing system is caused by the timer reaching the reduced timeout threshold.

6. The method according to claim 5, wherein, The timeout threshold is inversely correlated with the wind direction, and therefore the timeout threshold decreases in response to the wind direction deviating from the full wake direction.

7. The method according to any one of claims 4 to 6, wherein, The timeout threshold varies depending on the parameters related to wind variability, such that for a difference between the wind direction and the direction of the full wake, the timeout threshold increases with the increase of the parameters related to wind variability, up to at least the upper limit.

8. The method according to claim 7, wherein, Parameters related to wind variability are based on turbulence intensity parameters, standard deviation of wind speed, standard deviation of wind direction, or changes in blade load sensor signals.

9. The method according to claim 1, wherein, In response to the detection that the wind direction has crossed the full wake wind direction again before the timing system times out, the operation of the yaw offset controller is resumed.

10. The method according to claim 9, wherein, In response to the detection that the wind direction re-crosses the full wake wind direction before the timing system times out, the timing system is reset.

11. The method according to any one of the preceding claims, wherein, The timing system includes a first timer having a first timeout threshold and a second timer having a second timeout threshold shorter than the first timeout threshold, and the method includes: in response to detecting that the wind direction crosses the full wake wind direction: starting the first timer and maintaining the operation of the yaw offset controller; in response to detecting that the wind direction crosses the threshold before the first timer reaches the first timeout threshold, starting the second timer; and in response to the first timer reaching the first timeout threshold or the second timer reaching the second timeout threshold, resuming the operation of the yaw offset controller.

12. The method according to any one of the preceding claims, wherein, Detecting the wind direction crossing the full wake includes detecting the sign change of the yaw offset signal.

13. The method according to any one of the preceding claims, wherein, Resuming operation of the yaw offset controller causes the yaw offset controller to change the sign of the yaw offset signal.

14. The method according to any one of the preceding claims, wherein, The yaw offset signal is changed by the yaw offset controller based on the transfer function.

15. The method according to claim 14, wherein, The transfer function includes: a negative offset band, in which the yaw offset signal is negative and decreases to a minimum; a positive offset band, in which the yaw offset signal is positive and increases to a maximum; and transitions between the offset bands, in which the sign of the yaw offset signal changes.

16. A yaw offset controller configured to control a first wind turbine by means of any one of the preceding claims.

17. A wind turbine comprising: Rotor; Yaw control system; And the yaw offset controller according to claim 17.

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