Method of wake steering wind turbine

Abstract

A method of wake steering a first wind turbine. A second wind turbine is positioned downstream of the first wind turbine so that the second wind turbine can 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 create a yaw offset between a rotor of the first wind turbine and a wind direction, the yaw offset steering the wake away from the second wind turbine. A yaw offset controller is operated to vary the yaw offset signal in response to changes in the wind direction. In response to the detection of the wind direction crossing the full-wake wind direction a timing system is started and the operation of the yaw offset controller is held so that the yaw offset signal does not vary in response to changes in the wind direction. The operation of the yaw offset controller is reinstated in response to either: a timeout of the timing system, or detection of the wind direction re-crossing the full-wake wind direction before timeout of the timing system.

Description

[0001] METHOD OF WAKE STEERING WIND TURBINE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method of wake steering a wind turbine, a yaw offset controller configured to control a wind turbine by such a method; and a wind turbine comprising such a yaw offset controller.

[0004] BACKGROUND OF THE INVENTION

[0005] In a wind park comprising a plurality of wind turbines, a second wind turbine of the plurality of wind turbines can be affected by a wake region caused by a first wind turbine being is positioned upstream of the second wind turbine.

[0006] A known method of controlling a first wind turbine of a plurality of wind turbines of a wind park is disclosed in US2021 / 207580 A1. The method comprises: determining a current yaw state; determining a wind condition indicative of a level of potential wake inducement at the second wind turbine caused at least by the first wind turbine; and defining a rotor yaw offset angle set point for the first wind turbine based on the current yaw state, the wind condition, and at least one yaw angle hysteresis switching threshold. The rotor yaw offset angle set point follows a hysteresis, thereby avoiding immediate consecutive switching between a range of positive yaw offset angles and negative yaw offset angles.

[0007] It is of benefit to provide further ways of controlling wind turbines in a wind park experiencing wake influence from other wind turbines in the wind park.

[0008] SUMMARY OF THE INVENTION

[0009] A first aspect of the invention provides a method of wake steering a first wind turbine, wherein a second wind turbine is positioned downstream of the first wind turbine so that the second wind turbine can be affected by a 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 create a yaw offset between a rotor of the first wind turbine and a wind direction, the yaw offset steering the wake away from the second wind turbine; operating a yaw offset controller to vary the yaw offset signal in response to changes in the wind direction; detecting the wind direction crossing a full-wake wind direction, wherein the full-wake direction is parallel with a heading between the first and second wind turbines; in response to the detection of the wind direction crossing the full-wake wind direction: starting a timing system and holding the operation of the yaw offset controller so that the yaw offset signal does not vary in response to changes in the wind direction; and reinstating the operation of the yaw offset controller in response to a timeout of the timing system.

[0010] Optionally the timing system comprises a timer; the timing system is started by starting the timer, and the timeout of the timing system is caused by the timer reaching a timeout threshold.

[0011] Optionally the method further comprises varying the timeout threshold after the timer has been started.

[0012] Optionally the timeout threshold is reduced to a reduced timeout threshold in response to the wind direction deviating away from the full-wake direction; and the timeout of the timing system is caused by the timer reaching the reduced timeout threshold. The timeout threshold is inversely related to the wind direction, so it reduces in response to the wind direction deviating away from the full-wake direction.

[0013] Optionally, the timeout threshold is varied in dependency upon a parameter related to the variability of the wind so that the timeout threshold for a difference between wind direction and the full-wake direction is increased with increased parameter related to the variability of the wind, at least up to an upper limit. The parameter related to the variability of the wind may be based on a turbulence intensity parameter, based on a standard deviation of the wind speed, based on a standard deviation of the wind direction or based on a variation of a blade load sensor signal.

[0014] Optionally the timing system comprises a first timer with a first timeout threshold and a second timer with a second timeout threshold shorter than the first timeout threshold, and the method comprises: in response to the detection of the wind direction crossing the full-wake wind direction: starting the first timer and holding the operation of the yaw offset controller; starting the second timer in response to a detection of the wind direction crossing a breach threshold before the first timer reaches the first timeout threshold; and reinstating the operation of the yaw offset controller in response to the first timer reaching the first timeout threshold or the second timer reaching the second timeout threshold.

[0015] A second aspect of the invention provides a method of wake steering a first wind turbine, wherein a second wind turbine is positioned downstream of the first wind turbine so that the second wind turbine can be affected by a 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 create a yaw offset between a rotor of the first wind turbine and a wind direction, the yaw offset steering the wake away from the second wind turbine; operating a yaw offset controller to vary the yaw offset signal in response to changes in the wind direction; detecting the wind direction crossing a full-wake wind direction, wherein the full-wake direction is parallel with a heading between the first and second wind turbines; in response to the detection of the wind direction crossing the full-wake wind direction: starting a timing system and holding the operation of the yaw offset controller so that the yaw offset signal does not vary in response to changes in the wind direction; and reinstating the operation of the yaw offset controller in response to detection of the wind direction recrossing the full-wake wind direction before timeout of the timing system.

[0016] Optionally detecting the wind direction crossing the full-wake wind direction comprises detecting a change of sign of the yaw offset signal.

[0017] The following comments apply to either the first or second aspects of the present invention.

[0018] Optionally reinstating the operation of the yaw offset controller causes the yaw offset controller to change a sign of the yaw offset signal.

[0019] Optionally the yaw offset signal is varied by the yaw offset controller on the basis of a transfer function.

[0020] Optionally the transfer function comprises: 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 a transition between the offset bands in which the yaw offset signal changes sign.

[0021] A further aspect of the invention provides a yaw offset controller configured to control a first wind turbine by a method according to the first or second aspect.

[0022] A further aspect of the invention provides a wind turbine comprising: a rotor; a yaw control system; and a yaw offset controller according to the preceding aspect.

[0023] The yaw offset controller may be a control unit or control module, such as a unit or module comprised in a computer program. Such unit or module may be implemented to receive input signals, determine yaw offset signals, executing the method according to the first or second aspects of the invention, outputting / applying a yaw offset signal to the yaw control system of a wind turbine.

[0024] In an aspect, the invention further relates to a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to the first or second aspect.

[0025] The computer program may be a computer program product and may be in the form of a computer-readable storage medium or being downloadable from a communication network. The computer program comprises instructions, e.g. executable instructions in the form of software code, which when executed by a computer cause the control system of the wind turbine to carry out the method according to the first or second aspect.

[0026] The computer program may enable the yaw offset controller to carry out the instruction when loaded onto the data processing system of the control system. In general, the yaw offset controller may be a unit or collection of functional units which comprises one or more processors, input / output interface(s) and a memory capable of storing instructions that can be executed by a processor.

[0027] The yaw offset controller may be implemented as part of a control system of a wind turbine. Such control system may be a local control system arranged at the wind turbine or it may be a central control system arranged centrally and communicatively connected to the control system of the wind turbine, such as to the yaw control system and / or the yaw offset controller. The central control system may be arranged in one wind turbine and communicatively connected to another wind turbine via a communication network. The central control system may be arranged in a central control system, such as a SCADA control system or a power plant control system and communicatively connected to wind turbines via a communication network. The yaw offset controller may further be part of a distributed control system distributed among controller elements of a wind turbine control system, potentially including elements arranged local to a wind turbine as well as central controller elements.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0030] Figure 1 shows a wind turbine.

[0031] Figure 2A shows a control system of the wind turbine;

[0032] Figure 2B shows various elements of the yaw offset controller;

[0033] Figure 3 shows a transfer function;

[0034] Figure 4 shows an operating state at a first point of the transfer function;

[0035] Figure 5 indicates a second point of the transfer function;

[0036] Figure 6 shows an operating state at the second point;

[0037] Figure 7 indicates a third point of the transfer function;

[0038] Figure 8 shows an operating state at the third point;

[0039] Figure 9 indicates a fourth point of the transfer function;

[0040] Figure 10 shows an operating state at the fourth point;

[0041] Figure 11 shows wind direction and yaw offset signals for a long timer;

[0042] Figure 12 shows wind direction and yaw offset signals for a short timer;

[0043] Figure 13 shows elements of a yaw offset controller which is operating to vary a yaw offset signal in response to changes in the wind direction;

[0044] Figure 14 shows the yaw offset controller of Figure 13 in a held state;

[0045] Figure 15 is a first example of a timing system of the yaw offset controller of Figure 13; Figure 16 shows wind direction and yaw offset signals where the wind direction is oscillating around the full-wake direction;

[0046] Figure 17 shows an alternative timing system; Figure 18 shows a further alternative timing system;

[0047] Figure 19 shows a further alternative timing system;

[0048] Figure 20 is a graph showing the output of a timer with respect to time;

[0049] Figure 21 is a graph showing the output of a timer where a cross-back is detected;

[0050] Figure 22 is a graph showing how the output of a timer may vary in an alternative embodiment; and

[0051] Figure 23 is a graph showing how the output of a timer may vary in a further alternative embodiment; and

[0052] Figure 24 illustrates an embodiment where the timeout threshold is varied in dependency upon a parameter related to the variability of the wind.

[0053] DETAILED DESCRIPTION OF EMBODIMENT(S)

[0054] Figure 1 illustrates, in a schematic perspective view, a first wind turbine 1. The wind turbine 1 includes a tower 2 and a rotor-nacelle assembly (RNA) at the top of the tower 2. The RNA includes a nacelle 3 and a rotor 4 operatively coupled to a generator housed inside the nacelle 3. In addition to the generator, the nacelle 3 houses various components required for converting wind energy into electrical energy and various components needed to operate and control the wind turbine 1 .

[0055] The rotor 4 includes a central hub and a plurality of blades 5 that project outwardly from the central hub. In the illustrated wind turbine 1 , the rotor 4 includes three blades 5, but the number may vary.

[0056] Figure 2A schematically illustrates an embodiment of a control system 20 together with elements of the wind turbine 1. The rotor is mechanically connected to an electrical generator 7 via a gearbox 9 (in direct drive systems the gear box is not present). The electrical power generated by the generator 7 is injected into a power grid 24 via an electrical converter 25. The electrical generator 7 and the converter 25 may be based on a full scale converter (FSC) architecture or a doubly fed induction generator (DFIG) architecture, but other types may be used

[0057] The control system 20 comprises a number of elements, including at least one main controller 10 with a processor and a memory, so that the processor is capable of executing computing tasks based on instructions stored in the memory. In general, the main controller 10 ensures that in operation the wind turbine generates a requested power output level. This is obtained by adjusting the pitch angle of the blades and / or the power extraction of the converter 25. To this end, the control system 20 comprises 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 by a pitch mechanism. The rotor comprises an individual pitch system which is capable of individual pitching of the rotor blades 5, and may comprise a common pitch system which adjusts all pitch angles on all rotor blades at the same time. The control system 20, or elements of the control system 20, may be connected to a power plant controller (not shown) or other control system to receive externally provided instructions.

[0058] The wind turbine 1 comprises a yaw control system 31 configured to rotate the RNA 3,4 about a vertical yaw axis, in order to control a yaw angle of the rotor 4. The yaw control system 31 comprises various elements, including yaw motors. The control system 20 comprises a yaw offset controller 30 configured to generate a yaw offset signal 32 which is input to the yaw control system 31.

[0059] The yaw offset controller 30 is configured to control the wind turbine 1 by varying the yaw offset signal 32 in response to changes in the wind direction. The yaw offset signal 32 may be varied on the basis of a transfer function, which is stored in a memory of the yaw offset controller 30. One possible transfer function is explained in more detail below.

[0060] The yaw offset controller 30 is illustrated as part of the control system 20. In embodiments the yaw offset controller may alternatively be placed in a remote controller and communicatively connected to the control system of the wind turbine. Such remote controller may be a central controller implemented with the task of determining yaw offset setpoints for wind turbines in a wind park. The central controller may e.g. be implemented in a SCADA system communicatively connected to the wind turbines of the wind park. The central controller may alternative be implemented in a power plant control system arranged to control the wind turbines of the wind park. A central controller may form part of a distributed system comprising central elements, such as computational elements for calculating the yaw offset setpoint, and local elements, such as local controller elements arranged for receiving the yaw offset setpoints and controlling the yaw of the wind turbine.

[0061] Figure 2B gives a schematic overview of elements of the yaw offset controller 30 and part of the yaw control system 31. The yaw offset controller 30 comprises sensors and processing 80 and a yaw offset control module 81 ; and the yaw control system 31 comprises an upwind yaw control module 82.

[0062] The sensors 80 receive as inputs the wind and a calibrated nacelle heading; and output a wind direction and wind speed to the modules 81 , 82.

[0063] The yaw offset control module 81 outputs the yaw offset signal 32 to the upwind yaw control module 82. The yaw offset signal 32 is based on the wind direction, and the transfer function mentioned above, which may be stored by the yaw offset module 81 in the form of a look up table, or in any other way. As an alternative to, or in addition to, the look up table, the yaw offset signal may be calculated using a computer model arranged to simulate wake effects and interactions between turbines in a wind park.

[0064] The wind direction used by the yaw offset control module 81 may be an instantaneous wind direction, or a low pass filtered signal based on the wind direction.

[0065] The upwind yaw control module 82 outputs yaw motor activation signals to the yaw motors based on the yaw offset signal 32 (which gives a target yaw offset) and the wind direction and wind speed.

[0066] Figure 3 gives an example of a transfer function which may be used by the yaw offset controller 30.

[0067] The transfer function may be stored at the wind turbine 1 in a memory of the yaw offset controller 30. In other embodiments of the invention the transfer function may be stored remotely from the wind turbine 1 , for instance at a centralised location.

[0068] The transfer function may be stored in in the form of a look up table. In other embodiments of the invention, the transfer function may be obtained by an online optimization or similar. As shown in Figure 4, a second wind turbine 40 is positioned downstream of the first wind turbine 1 so that the second wind turbine 40 can be affected by a wake 41 of the first wind turbine 1. The direction between the turbines 1 , 40 is indicated by a heading 14. When the wind 13 is parallel with the heading 14 between the first and second wind turbines 1 , 40 the wind direction is referred to as a full-wake wind direction.

[0069] Figure 4 shows the wind direction and yaw angles of the wind turbines 1 , 40 at a first point 1a in the transfer function of Figure 3. For this wind direction +a, the yaw offset signal 32 is set to zero so the rotor axis 12 is parallel to the wind 13. The wind direction +a is defined here as the angle of the wind 13 relative to the heading 14.

[0070] Since the wind direction +a is a relatively large angle, the wake 41 does not significantly affect the second wind turbine 40 in the operating state of Figure 4.

[0071] As the wind direction reduces from +a, thereby moving closer to the full-wake wind direction, the transfer function causes a progressively increasing yaw offset to be applied until it reaches a maximum at the point 2a shown in Figures 5 and 6. For this wind direction +p, the yaw offset signal 32 is at a maximum +0 so the rotor axis is at an angle +0 to the wind as shown in Figure 6. As a result of the wake steering caused by the yaw offset, the wake 41 does not significantly affect the second wind turbine 40 in the operating state of Figure 6.

[0072] As the wind direction reduces from +p, the transfer function is flat until the point 3a shown in Figures 7 and 8 where the wind 13 is in the full-wake wind direction.

[0073] For this part of the transfer function the yaw offset signal 32 remains at the maximum +0 so the rotor axis remains at an angle +0 to the wind as shown in Figure 8. At point 3a, the wake 41 may affect the second wind turbine 40 as shown in Figure 8.

[0074] Next the transfer function has a step transition where it jumps from point 3a to point 4a indicated in Figure 9. The yaw offset signal 32 jumps to a minimum angle -0, so the rotor axis is at an angle -0 to the wind direction as shown in Figure 10. At point 4a, the wake 41 may affect the second wind turbine 40 as shown in Figure 10. The remaining parts of the transfer function of Figure 3 will not be described, since the transfer function is symmetrical.

[0075] Figure 11 is a graph with a line 50 indicating wind direction based on the left-hand scale. The wind direction is given in degrees, with the full-wake direction designated as a wind direction of zero degrees.

[0076] Lines 51 , 52 show two different yaw offset signals based on the right-hand scale. A yaw offset of +1 corresponds with angle +0 in the transfer function of Figure 3 and a yaw offset of -1 corresponds with angle -0 in the transfer function of Figure 3.

[0077] Figure 11 may illustrate a time period of a number of minutes, for instance 10 minutes or more. So by way of example a time of 1 on the time-scale of Figure 1 may be a time of 10 minutes. By way of non-limiting example the discussion below will assume that a time of 1 on the time axis of Figure 11 corresponds with a time of 10 minutes.

[0078] The wind direction 50 in Figure 11 crosses the full-wake direction at a time of about 3 minutes. The transfer function of Figure 3 has a step transition where it jumps from point 3a to point 4a, changing by an angle of 20 which may be of the order of 40deg. The vertical part of the dashed line 51 in Figure 11 shows the yaw offset signal immediately switching sign in response to the wind direction 50 crossing the full-wake direction.

[0079] The large size of this step transition can cause wear of the yaw control system 31 , particularly if the wind direction is switching back-and-forth rapidly across the full-wake wind direction. Figure 13 gives an example of how the yaw offset controller 30 can be modified to address the problem.

[0080] The yaw offset signal 32 is input to a zero-crossing detector 60, which detects a step transition where the yaw offset signal jumps between point 3a and point 4a (in either direction). The output of the zero-crossing detector 60 is input into 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 a modified yaw offset signal which is input into the yaw control system 31. In the period of time between 0 and 3 minutes in Figure 11 , the Boolean output of the timing system 61 is zero, and the switch 62 is in the position shown in Figure 13, so the modified yaw offset signal is the same as the un-modified yaw offset signal 32.

[0081] When the zero-crossing detector 60 detects a step transition at 3 minutes (caused by the wind direction becoming negative) the Boolean output of the timing system 61 becomes non-zero and the switch 62 switches to the position shown in Figure 14 so the modified yaw offset signal is based on a 1 / z input. As a result, the modified yaw offset signal does not follow the step transition of the yaw offset signal 32, but instead retains its current value as indicated by dotted line 52 in Figure 11. Hence the operation of the yaw offset controller 30 is held in response to the detection of the wind direction crossing the full-wake wind direction, so that the modified yaw offset signal 52 does not vary in response to changes in the wind direction.

[0082] The timing system 61 is also started in response to the detection of the wind direction crossing the full-wake wind direction. In the example of Figure 11 , the timing system 61 comprises a long timer, which is started in response to the detection of the wind direction crossing the full-wake wind direction, and times out in 4 minutes.

[0083] During the hold time of the timing system 61 (i.e. before the timeout of the timing system 61) the yaw offset signal before switching is retained. The timeout of the timing system 61 causes its Boolean output to return to zero, so the switch 62 returns to the position of Figure 13. Hence the operation of the yaw offset controller 30 is reinstated in response to the 4 minute timeout of the timing system 61. In the example of Figure 11 , reinstating the operation of the yaw offset controller 30 causes the yaw offset controller to change a sign of the yaw offset signal. That is, the yaw offset signal 52 changes sign at a time of about 7 minutes, because at this time the wind direction is still negative.

[0084] In this example the yaw offset signal 52 jumps by two (i.e. an angular change of 20) because at the time of reinstatement the wind direction is between -p and zero, and hence in the flat part of the transfer function of Figure 3.

[0085] If the wind direction is less than - at the time of reinstatement of the operation of the yaw offset controller, then the yaw offset signal 52 will change sign but jump by less than two, depending on the point in the transfer function. If the wind direction is positive at the time of reinstatement of the operation of the yaw offset controller, then the yaw offset signal 52 will not change sign.

[0086] Figure 12 shows a dotted line 53 for a modified yaw offset control signal with a long timer (with a 4 minute timeout) and a dotted line 54 for a modified yaw offset control signal with a short timer (with a 3 minute timeout).

[0087] Figure 15 gives an example of the timing system 61 , comprising a latch 70 which is triggered by the output of the zero crossing detector 60. The output of the latch 70 is the Boolean output of the timing system 61.

[0088] When the Boolean output of the latch 70 goes true, a timer 71 starts counting. The output of the timer 71 is subtracted from a timeout threshold 73 by a subtractor 72. When the output of the subtractor 72 goes negative, the latch 70 is released and the Boolean output of the timing system 61 switches, reinstating the operation of the yaw offset controller by operating the switch 62. The release of the latch 70 also re-sets the timing system 61 by re-setting the timer 71 to zero.

[0089] In a simple example, the timeout threshold 73 may be fixed, for example 3 minutes or 4 minutes in the examples of Figure 12.

[0090] 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 ramp 75 which 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 further from the full-wake direction. Note that the timeout threshold is inversely related to the wind direction, so it reduces in response to the wind direction deviating away from the full-wake direction.

[0091] After the timer 71 starts counting, the function 74 may cause the timeout threshold 73 to be reduced to a reduced timeout threshold in response to the wind direction deviating away from the full-wake direction. For example the timeout threshold 73 may reduce from the high value 76 to the low value 77, or to an intermediate value at some point on the linear ramp 75. The timeout of the timing system 61 is caused by the timer reaching the reduced timeout threshold, which may vary continuously based on the function 74.

[0092] Figure 16 illustrates a time period in which the wind direction crosses the full-wake direction four times, as indicated by solid line 50. In a conventional system this would result in a rapid switching back-and-forth of the yaw offset signal indicated by the dashed line 51 in Figure 15.

[0093] In the example of Figure 16, despite the wind direction crossing the full-wake direction four times, the yaw offset signal output to the yaw control system 31 remains constant, as indicated by dotted line 52.

[0094] The first zero-crossing at about 1.5 minutes causes the timer 71 to start counting. The wind direction re-crosses the full-wake wind direction at about 3 minutes, before timeout of the timing system 61.

[0095] Figure 17 shows an alternative timing system 61 which includes a re-crossing detection signal 78 which releases the latch 70 in response to the wind direction re-crossing the full-wake wind direction at about 3 minutes, before timeout of the timing system 61. The release of the latch 70 also re-sets the timing system 61 by re-setting the timer 71 to zero. At the next zero-crossing in Figure 16, just before 6 minutes, the latch 70 is triggered again.

[0096] The re-crossing detection signal 78 may be generated by detecting a change of sign of the yaw offset signal, or by detecting the wind direction crossing the full-wake wind direction in some other way.

[0097] Figure 18 shows an alternative timing system 61 which achieves a similar effect to the alternative timing system of Figure 17, but without a re-crossing detection signal 78. In this case the function 74 is modified to include a step transition 79 (at an angle of zero) where the timeout threshold 73 switches from positive to negative. This causes the timing system 61 to immediately timeout, releasing the latch 70 and re-setting the timing system by re-setting the timer 71 to zero. The step transition 79 has a similar effect to the re-crossing detection signal 78, i.e. it releases the latch 70 in response to the wind direction re-crossing the full-wake wind direction before timeout of the timing system 61. At the next zero-crossing in Figure 16, just before 6 minutes, the latch 70 is triggered again.

[0098] The function 74 in Figures 17 and 18 may be piecewise continuous and piecewise differentiable.

[0099] Figure 19 shows a further alternative timing system 61. In this case two timers are provided: a first timer 71a with a first (long) timeout threshold (for instance 5-10 minutes) and a second timer 71b with a second (short) timeout threshold (for instance 1-2 minutes) shorter than the first timeout threshold.

[0100] When the Boolean output of the latch 70 goes true, the first timer 71a starts counting. The output of the first timer 71a is subtracted from the long timeout threshold by a subtractor 72a.

[0101] When the wind direction has deviated away from the full-wake direction by a breach threshold, before the first timer 71a reaches the first timeout threshold, the second timer 71 b starts counting. The output of the second timer 71b is subtracted from the short timeout threshold by a subtractor 72b.

[0102] Timeout of the timing system 61 occurs when the output of either of the subtractors 72a, 72b goes negative. This causes the latch 70 to be released and the Boolean output of the timing system 61 to switch, reinstating the operation of the yaw offset controller by operating the switch 62. The release of the latch 70 also re-sets the timing system 61 by re-setting the timers 71a, 71 b to zero.

[0103] Hence in Figure 19 the operation of the yaw offset controller is reinstated in response to the first timer 71a reaching the first timeout threshold or the second timer 71b reaching the second timeout threshold.

[0104] In Figure 19, the timing system 61 has two timers 71a, 71 b with different timeout thresholds. The number of timers is not limited to two, so in other embodiments the timing system 61 may have three or more timers, each with a different timeout threshold. Figure 20 is a graph with a line 100 showing the output of the timer 71 of any of the previous embodiments with respect to time. The output of the timer counts up from zero until each it reaches the timeout threshold 101. When it reaches the timeout threshold 101 , the operation of the yaw offset controller is reinstated and the timer is re-set.

[0105] Figure 21 is a graph showing the output of the timer 71 in the embodiment of Figure 17 or 18 where a cross-back is detected. The output of the timer counts up from zero until a cross-back detection at time T 1 , i.e. a detection of the wind direction re-crossing the full-wake wind direction. In response to the cross-back detection at time T1 , the timing system 61 is re-set (as indicated by vertical line 102) and the operation of the yaw offset controller is immediately reinstated.

[0106] Figure 22 is a graph showing how the output of the timer may vary in an alternative embodiment. The output of the timer counts up from zero until a cross-back detection at time T 1 , i.e. a detection of the wind direction re-crossing the full-wake wind direction. In response to the cross-back detection at time T1 , the timer starts counting down until it reaches zero at time T2, causing a timeout of the timing system 61. In response to the timeout of the timing system 61 at time T2, the operation of the yaw offset controller is reinstated. Note that in other examples, the timeout of the timing system 61 is caused by the output of the timer reaching a threshold. In this example, the timeout of the timing system 61 is caused by the output of the timer returning to zero at time T2.

[0107] Figure 23 is a graph showing how the output of the timer may vary in a further alternative embodiment. The output of the timer counts up from zero until a cross-back detection at time T 1 , i.e. a detection of the wind direction re-crossing the full-wake wind direction. In response to the cross-back detection at time T1 , the timer starts counting down until another cross-back detection at time T2 before the timer reaches zero. After time T2, the output of the timer starts counting up again until each it reaches the timeout threshold 101. When it reaches the timeout threshold 101 at time T3, the operation of the yaw offset controller is reinstated and the timer is re-set.

[0108] The timeout threshold 73 may in embodiments vary based on the function 74 of the angle between the wind direction and the full-wake direction, such as based on a difference (angular difference) between wind direction and the full-wake direction. In the illustrated embodiments, the function 74 has a linear ramp 75 which 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 further from the full-wake direction. Above a minimum difference between wind direction and the full-wake direction, the timeout threshold being inversely related to the difference between wind direction and the full-wake direction, so it reduces in response to the wind direction deviating away from the full-wake direction.

[0109] The function 74 varies the timeout threshold as a function of wind direction, but in other embodiments of the invention the timeout threshold may be varied as a function of other parameters, such as a parameter related to the variability of the wind, hereunder based on a turbulence intensity parameter, based on a standard deviation of the wind speed, a standard deviation of the wind speed direction or based on a variation of a blade load sensor signal.

[0110] Figure 24 illustrates an embodiment of the function 74 where the timeout threshold is varied in further dependency upon a parameter related to the variability of the wind so that the timeout threshold for a same difference between wind direction and the fullwake direction is increased 79 with increased parameter related to the variability of the wind, at least up to an upper limit and at least until a minimum threshold. The upper limit being illustrated with the curve segments 76C, 75C and 77.

[0111] For example, with increased turbulence intensity, Tl, the piecewise linear threshold is moved to higher values from a base curve 75, 76 to a maximal curve 75C, 76C passing one or more intermediate curves 75B, 76B. The curves have same low value 77, but reach the low value along different piecewise linear curves.

[0112] The turbulence intensity may be defined in different ways, hereunder as the standard deviation of the windspeed divided with the average wind speed, the standard deviation of the relative wind direction divided with the average wind speed, or as the standard deviation of the relative wind direction.

[0113] The timeout threshold may e.g. be set as a function of the turbulence intensity and the wind direction difference function so that for a difference between wind direction and the full-wake direction being below a minimum value, 78, a constant timeout threshold 76, 76B, 76C is set, the constant timeout threshold increasing 79 with increasing turbulence intensity from the base timeout threshold 76 to a maximum timeout threshold 76C. For a difference between wind direction and the full-wake direction being above the minimum 78, the timeout threshold reduces (piecewise) linearly with increasing difference, until a certain difference where the timeout threshold reaches a minimum value 77. In addition to the decrease as a function of the difference, the timeout threshold increasing 79 with increasing turbulence intensity from the base timeout threshold 76 to a maximum timeout threshold 76C. The curves may be implemented in different manners, e.g. based on a function setting the constant level in the region below the minimum difference between wind direction and the full-wake direction, and as functions setting the slope in the region above the minimum difference between wind direction and the full-wake direction. Further and different ways of implementing varying the timeout threshold as a function of various parameters may be devised by the skilled person.

[0114] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1. A method of wake steering a first wind turbine, wherein a second wind turbine is positioned downstream of the first wind turbine so that the second wind turbine can be affected by a 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 create a yaw offset between a rotor of the first wind turbine and a wind direction, the yaw offset steering the wake away from the second wind turbine; operating a yaw offset controller to vary the yaw offset signal in response to changes in the wind direction; detecting the wind direction crossing a full-wake wind direction, wherein the full-wake direction is parallel with a heading between the first and second wind turbines; in response to the detection of the wind direction crossing the full-wake wind direction: starting a timing system and holding the operation of the yaw offset controller so that the yaw offset signal does not vary in response to changes in the wind direction; and reinstating the operation of the yaw offset controller in response to either: a timeout of the timing system, or detection of the wind direction re-crossing the full-wake wind direction before timeout of the timing system.

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

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

4. A method according to claim 3, further comprising varying the timeout threshold after the timer has been started.

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

6. A method according to claim 5, wherein the timeout threshold is inversely related to the wind direction, so it reduces in response to the wind direction deviating away from the full-wake direction.

7. A method according to claim 4 to 6, wherein the timeout threshold is varied in dependency upon a parameter related to the variability of the wind so that the timeout threshold for a difference between wind direction and the full-wake direction is increased with increased parameter related to the variability of the wind, at least up to an upper limit.

8. A method according to claim 7, wherein the parameter related to the variability of the wind is based on a turbulence intensity parameter, based on a standard deviation of the wind speed, based on a standard deviation of the wind direction or based on a variation of a blade load sensor signal.

9. A method according to claim 1 , wherein the operation of the yaw offset controller is reinstated in response to detection of the wind direction re-crossing the full-wake wind direction before timeout of the timing system.

10. A method according to claim 9, wherein the timing system is re-set in response to the detection of the wind direction re-crossing the full-wake wind direction before timeout of the timing system.

11. A method according to any preceding claim, wherein the timing system comprises a first timer with a first timeout threshold and a second timer with a second timeout threshold shorter than the first timeout threshold, and the method comprises: in response to the detection of the wind direction crossing the full-wake wind direction: starting the first timer and holding the operation of the yaw offset controller;starting the second timer in response to a detection of the wind direction crossing a breach threshold before the first timer reaches the first timeout threshold; and reinstating the operation of the yaw offset controller in response to the first timer reaching the first timeout threshold or the second timer reaching the second timeout threshold.

12. A method according to any preceding claim, wherein detecting the wind direction crossing the full-wake wind direction comprises detecting a change of sign of the yaw offset signal.

13. A method according to any preceding claim, wherein reinstating the operation of the yaw offset controller causes the yaw offset controller to change a sign of the yaw offset signal.

14. A method according to any preceding claim, wherein the yaw offset signal is varied by the yaw offset controller on the basis of a transfer function.

15. A method according to claim 14, wherein the transfer function comprises: 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 a transition between the offset bands in which the yaw offset signal changes sign.

16. A yaw offset controller configured to control a first wind turbine by a method according to any preceding claim.

17. A wind turbine comprising: a rotor; a yaw control system; and a yaw offset controller according to claim 17.

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