Enhanced wind turbine wake mixing

Abstract

A method for controlling a wind turbine is disclosed, the wind turbine including a rotor including at least a first blade, the method comprising the steps of: dynamically changing the pitch angle of the first blade over time between a first pitch angle and a second pitch angle while the first blade rotates, thereby changing the induction factor of the first blade over time, wherein the first pitch angle is different from the second pitch angle, and wherein controlling the dynamic change of the pitch angle over time causes the first blade to be displaced in a timely manner at a corresponding rotational position of the first blade at the first pitch angle and at a corresponding rotational position of the first blade at the second pitch angle, and changing the induction factor of the blade over time occurs at different angular positions in the rotor plane, thereby causing the position and / or direction of the wake formed downstream of the wind turbine to change dynamically relative to the rotor of the wind turbine.

Description

Technical Field

[0001] The present invention relates to a method for controlling a wind turbine, a wind turbine controller arranged in the method for controlling a wind turbine, a wind turbine including the wind turbine controller arranged in the method for controlling a wind turbine, and a wind turbine array, wherein at least a first wind turbine includes a wind turbine controller. Background Technology

[0002] To meet the 1.5-degree Celsius climate stability target set forth in the Paris Agreement, a significant reduction in fossil fuel power generation is needed in the coming years. To replace this power generation capacity, a substantial increase in renewable energy sources such as solar and wind power is required.

[0003] The most efficient way to develop wind energy on a large scale is by placing individual wind turbines on land or at sea in so-called wind farms. These wind farms consist of multiple wind turbines, usually of the same type, separated by a certain area and often sharing common infrastructure, thereby reducing the overall capital operating costs of the turbines and allowing for efficient maintenance of the wind farm while limiting the use of land and / or sea areas.

[0004] However, when a wind turbine extracts energy from the wind, a wake is generated downstream of the turbine. In the wind turbine wake, the (average) wind speed decreases and the (average) turbulence increases. This adverse effect is felt further downstream, as the reduced wind speed results in lower energy output, and the increased turbulence increases the fatigue load on the downstream turbine.

[0005] As the flow travels further downhill, the wake diffuses and mixes with the surrounding free-flowing wind, causing the wake to revert to a free-flowing state over time (and thus with distance). Since wind farm layout involves trade-offs between available area, installed capacity, and infrastructure costs, spacing wind turbines at distances that minimize wake effects is economically unattractive.

[0006] To optimize power output at the wind farm level, unlike individual wind turbine levels, wind farm control research focuses on steady-state optimization control, also known as axial-induced control or derating methods. This involves controlling the wind turbines located upstream of the wind farm to reduce their power output (i.e., derate their performance), allowing the downwind turbines to extract slightly more energy from the wind passing over them. The goal is to find optimal static control parameters regardless of wind and turbine dynamics. However, it has been found that this static approach can potentially lead to even lower total power output compared to a steady-state "greedy" strategy where all turbines operate at their individual optimal levels. Summary of the Invention

[0007] The objective of this invention (and possibly others) may be to obtain a method for controlling a wind turbine that reduces the wake effect downstream of the wind turbine, which at least partially alleviates one of the aforementioned problems.

[0008] Among other objectives, this objective can be met at least in part by a method of controlling a wind turbine including a rotor comprising at least a first blade, the method comprising the steps of: while the first blade rotates, changing the induction factor, particularly the radial induction factor, of the first blade over time by dynamically changing the pitch angle of the first blade over time between a first pitch angle and a second pitch angle, wherein the first pitch angle is different from the second pitch angle, and wherein the dynamic change of the pitch angle over time causes the blade to be timely displaced at the corresponding rotational position of the first blade at the first pitch angle and the second pitch angle, such that the position and / or direction of the wake formed downstream of the wind turbine changes dynamically relative to the rotor of the wind turbine.

[0009] In another aspect of the invention, instead of dynamically changing the position and / or direction of the wake formed downstream of the wind turbine relative to the rotor of the wind turbine, the dynamic change of the pitch angle over time can also be controlled so that the shape (rather than the position and / or direction) of the wake formed downstream of the wind turbine dynamically changes relative to the rotor of the wind turbine. The changing shape may, for example, correspond to a wake having a substantially constant shape that rotates along the axis of the rotor over time, preferably at a speed (significantly) lower than the rotor speed.

[0010] A wind turbine includes a rotor comprising at least one blade, but typically multiple blades, such as two or three, to convert the kinetic energy of wind into mechanical energy, which is then converted into electrical energy by a generator. Specifically, wind speed causes the blades to rotate, which in turn powers the generator. However, the rotating blades (effectively forming a rotor disk) slow the wind and result in a wake behind the turbine. The wind within the wake has a slower average velocity and higher average turbulence than the wind passing around the rotor disk that does not penetrate the turbine blades. Furthermore, the slower wind speed in the wake relative to the wind speed unaffected by the rotor causes the wake to expand, i.e., its diameter expands beyond the diameter of the rotor. Typically, the wake defines the volume of fluid (e.g., air) affected by the turbine blades. Any turbine positioned downstream (downwind) of this wake can only use relatively slower wind speeds to rotate its blades, resulting in lower power output from the downwind turbine. Additionally, the downwind turbine will also experience more fatigue loads due to the increased average turbulence, adversely affecting its lifespan.

[0011] By transferring kinetic energy from the wind surrounding the wake into the wake itself, the relative velocity and turbulence difference between the wind in the wake and the surrounding air slowly decrease over time. This process is called turbulent mixing. Because turbulent mixing occurs naturally, the kinetic energy transferred from the wind to the wind turbine will eventually be replaced. However, the distance required to transfer kinetic energy into the wake depends on the wind speed. Therefore, the distance between a first turbine and a second turbine positioned adjacent to each other (typically less than 10 rotor diameters, or 10D) may not be sufficient to return the kinetic energy captured by the first turbine before the wind reaches the second turbine located downstream of the first wind turbine.

[0012] Typically, the induction factor is determined by the wind speed V upstream of the rotor plane. ∞ The wind speed V at the rotor plane (i.e., the rotor disk) d The difference divided by the upstream wind speed V ∞ To determine, such that:

[0013]

[0014] Furthermore, each blade can have its own individual induction factor. The induction factor of a blade can be changed by pitching the blade relative to the wind (i.e., by rotating the blade about its longitudinal axis, causing a change in the angle between the blade's cross-section and the rotor plane). By changing the induction factor of the blade, the speed and direction of the wind leaving the rotor plane can be locally altered, thus effectively changing the position of the wake itself.

[0015] By timely displacing the blade at the corresponding rotational position of the first blade at the first pitch angle and preferably at the second pitch angle, the induced changes in the blade (i.e., the inducing factors of change) occur over time at different angular positions in the rotor plane. Therefore, the position of the wake formed downstream of the wind turbine also changes dynamically relative to the wind turbine rotor. This change in wake position increases turbulent mixing, reduces the distance required to transfer kinetic energy into the wake, and makes any turbine positioned downstream of the wind turbine much less affected by the wake. In the case of multiple blades, at least the pitch angle of the first blade is preferably arranged to change independently relative to the other blades.

[0016] In an alternative embodiment, the method includes the steps of applying yaw and tilting moments to the rotor to achieve forced wake mixing by superimposing periodic variations of the pitch angle onto the pitch angle of the first (e.g., common) blade of the wind turbine to dynamically change the pitch angle of the first blade. Therefore, the advantage of this method is that improved wake mixing can be achieved by applying only minor changes to existing control methods for wind turbines (i.e., by superimposing periodic variations onto the pitch angle).

[0017] In an alternative embodiment of the method, the step of changing the induction factor of the first blade over time further includes: dynamically changing the pitch angle of the first blade according to a predetermined periodic function, wherein the predetermined periodic function is defined such that the timely displacement blade is at the corresponding rotational position of the first blade at the first pitch angle and the second pitch angle. Periodic variation (i.e., according to the predetermined periodic function) is a simple and effective way to ensure that the timely displacement blade is at the corresponding rotational position of the first blade at the first pitch angle and the second pitch angle, thereby enabling dynamic changes in the position and / or direction of the wake formed downstream of the wind turbine.

[0018] In an alternative embodiment of the method, a predetermined periodic function is defined such that the rotational position of the blade in the rotor plane changes from rotation to rotational displacement, at which the blade is at a first pitch angle. Due to the induced changes in the blade at different angular positions in the rotor plane, the combined thrust acting on the blade will also change with the induced changes, causing the entire rotor to experience periodic changes in the orientation of the thrust without significantly altering the magnitude of the force itself. Therefore, only relatively small changes in thrust are experienced, so that the thrust does not significantly increase the induced fatigue load on the turbine. In particular, if the displacement from rotation to rotation is relatively slow, for example, less than 180° per rotation, preferably less than 90° per rotation, more preferably less than 45° per rotation, then the periodic change in thrust orientation is a low-frequency force change that does not lead to a significant increase in turbine fatigue load.

[0019] In an alternative embodiment, the rotor includes a second blade, and the method includes the step of: changing the induction factor of the second blade over time by dynamically changing the pitch angle of the second blade between a first pitch angle and a second pitch angle, wherein the time the first blade is at the first pitch angle differs from the time the second blade is at the first pitch angle. Changing the pitch angle of the first blade may cause some imbalance in the rotor. By also dynamically changing the pitch angle of the second blade as described herein, the imbalance can be at least partially compensated.

[0020] In an alternative embodiment, the rotor includes a second blade, preferably arranged such that its pitch angle can be changed individually or independently of the pitch angle of the first blade, and wherein the method preferably includes the steps of: dynamically changing the induction factor of the second blade over time by changing the pitch angle of the second blade according to a predetermined periodic function, and wherein the dynamic change of the pitch angle of the second blade differs from the dynamic change of the pitch angle of the first blade by a phase shift. By also changing the induction factor of the second blade according to a predetermined periodic function, but with a phase shift relative to the first blade, the pitch angles of the first and second blades (and therefore their induction factors) are not simultaneously at their maximum or minimum values. If the induction factors are simultaneously at their maximum or minimum values, the turbine will be effectively derated, and its power output will be reduced, because the rotor's induction factor will be changed as a total, rather than producing a local variation, which would have only a small effect on the total rotor's induction factor and therefore on the turbine's power output.

[0021] The phase offset can be chosen to be approximately equal to the cross angle between the first and second blades in the rotor plane. For example, for a two-bladed turbine, the cross angle is approximately 180°. By also having a phase offset of approximately 180°, the minimum inducible factor of the first blade is simultaneously compensated by the maximum value of the second blade, minimizing the change in the total inducible factor for the entire rotor. For example, for a three-bladed turbine, the cross angle is approximately 120°. By also having a phase offset of approximately 120°, the minimum inducible factor of the first blade is approximately compensated by the other two blades.

[0022] According to an alternative embodiment of the method, the dynamic change of the pitch angle over time is obtained by performing an inverse multi-blade coordinate (MBC) transformation on a time-varying yaw function defined in a non-rotating reference coordinate system, or on a time-varying tilt function defined in a non-rotating reference coordinate system, or on a combination of time-varying yaw and tilt functions. Multi-blade coordinates (MBC) are commonly used to transform the torque on the blades from a local blade coordinate system to a non-rotating or ground-fixed inertial coordinate system to determine, for example, loads on a turbine tower. By defining time-varying yaw and / or tilt signals capable of manipulating the wake horizontally and / or vertically respectively in a non-rotating coordinate system, and performing an inverse MBC transformation, the signals are transformed to a local blade coordinate system, thereby obtaining the dynamic change of the pitch angle of at least the first blade.

[0023] In embodiments of the invention, the time-varying yaw function is a periodic yaw function and / or the time-varying tilt function is a periodic tilt function. A predetermined periodic function is obtained by performing an inverse multi-blade coordinate (MBC) transformation, such that dynamically changing the pitch angle over time is based on the predetermined periodic function. Therefore, relatively simple functions can be implemented to achieve the desired effect described above. Preferably, the periodic tilt function and / or the periodic yaw function are sine functions with a predetermined frequency. Thus, the inverse transformation results in a periodic pitch function for a single blade, which is also a sine function or a superposition of sine functions, resulting in a smooth pitch signal. Due to the size and weight of the individual blades, especially in practical-scale wind turbines, a smooth pitch signal is preferred because it avoids introducing sudden, shock-like excitations through the pitch mechanism, which would induce all kinds of unwanted dynamics in the wind turbine structure and introduce increased loads on the turbine and its components.

[0024] In one embodiment of the method, the predetermined periodic function includes a first sine function having a first frequency, wherein the first frequency is different from the rotor's rotational frequency or a multiple thereof. Alternatively, the predetermined periodic function includes a superposition of a first sine function and a second sine function having a second frequency, wherein the first frequency and the second frequency are different. As described above, the sine function or the superposition of sine functions provides a smooth, periodic pitch angle variation.

[0025] The value of the first frequency or the second frequency can be chosen to be approximately equal to the rotor's rotational frequency, which increases or decreases with a predetermined frequency, and is a non-zero frequency less than the rotational frequency. Thus, the dynamic change in the pitch angle of at least the first blade according to a predetermined periodic function is relatively slow compared to the turbine's rotational frequency. Therefore, these low-frequency signals provide a slow and smooth periodic pitch angle change that is not expected to lead to a significant increase in load on the turbine, while simultaneously achieving a relatively slow change in the position and direction of the wake that leads to increased wake mixing. Since the predetermined frequency is a non-zero frequency less than the rotational frequency, the effect of improved wake mixing is achieved as described above, while the blade pitching motion is only slightly increased compared to existing individual pitch control methods, for example, used to reduce loads, thus limiting additional loads on the pitch system, especially on the pitch bearings, which are typically the most fatigue-critical part of the pitch system.

[0026] Preferably, the predetermined frequency is determined at least based on the rotor diameter, the rotor speed, and / or the inflow wind speed determined upstream of the wind turbine. Therefore, the predetermined periodic function is tailored to accommodate different operating conditions or turbine sizes, allowing for increased wake mixing for different operating conditions and turbine sizes. Alternatively or additionally, the predetermined frequency is preferably determined at least based on the Struhal number, wherein the Struhal number is preferably between 0.05 and 1.0, more preferably between 0.15 and 0.55, and even more preferably between 0.2 and 0.3, most preferably around 0.25.

[0027] The predetermined frequencies of the periodic tilt and / or yaw functions can be determined using a dimensionless number called the Struhal number:

[0028]

[0029] The dimensionless number limits the inflow wind speed U. ∞ The relationship between the turbine rotor diameter D and the predetermined frequency f is discussed. Based on computer simulations using a simulation program (Simulator for Wind Farm Applications (SOWFA)), the optimal Struhal number is estimated to be preferably between 0.05 and 1.0, more preferably between 0.15 and 0.55, even more preferably between 0.2 and 0.3, and most preferably around 0.25, for different frequencies under laminar flow conditions. It has been found that applying any embodiment to the predetermined frequency determined according to this Struhal number yields excellent wake mixing.

[0030] In an alternative embodiment of the method, the difference between the first pitch angle and the second pitch angle is 30° or less, preferably 20° or less, more preferably 10° or less, and most preferably between 2° and 8°. Too large a change in pitch angle will lead to a reduction in turbine performance, while too small a change will not result in the required amount of wake mixing. A good trade-off between these two is found within the given range.

[0031] In a second aspect of the invention, a wind turbine controller is provided, arranged for controlling a wind turbine including a rotor comprising at least a first blade. The controller is arranged to change the induction factor of the first blade over time by dynamically changing the pitch angle of the first blade according to a predetermined periodic function, such that the pitch angle of the first blade periodically varies between a first pitch angle and a second pitch angle as the first blade rotates. The first pitch angle is different from the second pitch angle, and the predetermined periodic function is defined such that the corresponding rotational positions of the first blade at the first and second pitch angles are timely displaced. This allows the controller to dynamically change the position of the wake formed downstream of the wind turbine relative to the rotor of the wind turbine. Thus, the advantages of the control method are applied to the controller.

[0032] In a third aspect of the invention, a wind turbine is provided, including a rotor comprising at least a first blade, and a wind turbine controller arranged for a method of controlling the wind turbine according to any of the given embodiments. Thus, a wind turbine is obtained that is capable of improving mixing in the wake formed downstream of the turbine.

[0033] In another aspect, an array of at least two wind turbines is provided, wherein, for a given wind direction, a second wind turbine is at least partially arranged downstream of the wake of a first wind turbine, wherein both the first and second wind turbines include rotors comprising at least a first blade, and wherein at least the first wind turbine includes a wind turbine controller arranged for a method of controlling the wind turbines according to any of the given embodiments. Thus, a turbine array (e.g., a wind farm) is obtained, wherein at least one turbine is configured to improve mixing in the wake formed downstream of the turbine, thereby further increasing the electrical energy generation of the wind turbine array (i.e., the wind farm). Attached Figure Description

[0034] The present invention is further illustrated by the following figures, which illustrate exemplary embodiments of a method for controlling a wind turbine according to the present invention and are not intended to limit the scope of the invention in any way, in which:

[0035] - Figure 1A A horizontal-axis wind turbine comprising a three-bladed rotor is schematically shown.

[0036] - Figure 1B The diagram schematically shows the pitching blades.

[0037] - Figure 2 The diagram schematically shows the nacelle and rotor of a wind turbine, which includes different components.

[0038] - Figure 3 An array of two wind turbines is schematically shown, wherein the second wind turbine is positioned downstream of the wake of the first wind turbine.

[0039] - Figure 4 A flowchart illustrating the different steps included in an embodiment of a method for controlling a wind turbine is shown.

[0040] - Figure 5The graphs show the average wake velocities at different distances after the turbine, controlled based on different Strouhal numbers. When greedy control is applied, the velocities are normalized by dividing by the wake velocities at each location.

[0041] - Figure 6A and Figure 6B The diagram schematically illustrates the position of the wake at different times during an excitation cycle T for two different embodiments of the method controlled respectively.

[0042] - Figure 7 The difference between the wake generated by a turbine controlled using a greedy control method and the wake generated by a turbine controlled using an embodiment of the method for controlling a wind turbine according to the present invention is shown. Detailed Implementation

[0043] Figure 1A The layout of a typical three-bladed horizontal-axis wind turbine 1 is schematically shown. The wind turbine includes a tower 2 located atop a base 3. Note that such a wind turbine can be deployed on land (e.g., onshore) and at sea (e.g., offshore). In the latter case, the base 3 will typically be an offshore base, such as a seabed-fixed structure (e.g., monopile, tripod, jacket) mounted to the seabed, or alternatively, a floating base in which a buoy is secured to the seabed to hold it in place. In the case of an onshore turbine, this base 3 is typically a so-called gravity base, which comprises a heavy concrete mass to hold the wind turbine 1 anchored to the ground.

[0044] The nacelle 4, connected to the rotor 5, is located at the top of the tower 2. The rotor 5 comprises three blades 51, 52, and 53, but any number of blades is possible; for example, one, two, or four blades may be used. Blades 51, 52, and 53 are fixed to the hub 54. The rotation of the nacelle 4 about a vertical axis I that is substantially parallel or coincident with the tower 2 and substantially perpendicular to the ground plane is called yaw rotation. The yaw angle can be defined according to the wind direction; in this case, a non-zero yaw angle means that there is a misalignment between the direction of the rotor axis II and the wind direction W. The rotor 5 is arranged to rotate about the rotor axis II; this rotation is commonly referred to as azimuth rotation. Furthermore, the blades 51, 52, and 53 are arranged to rotate about their respective longitudinal axes III; this rotation is called pitch rotation, and the angle between the central axis V of the cross-section of the blades 51, 52, and 53 and the plane of rotation IV of the rotor 5 is called the pitch angle. Figure 1B The central axis V of the cross section of the first blade 51, which is pitched relative to the rotor plane IV at a pitch angle θ1, is shown.

[0045] Figure 2 The nacelle 4 and rotor 5 of a wind turbine 1 are schematically shown, with different components arranged within the nacelle 4. The nacelle 4 houses a drivetrain 6, which may include a generator 61 for generating electrical energy, and a gearbox 62 arranged between a high-speed shaft 63 and a low-speed shaft 64. The low-speed shaft 64 is connected to the rotor 5, and the high-speed shaft 63 transmits rotation from the output of the gearbox 62 to the generator 61. Note that in so-called direct-drive wind turbines, the rotor is typically directly connected to the generator via a main shaft or a low-speed shaft. In these types of wind turbines, the gearbox 62 and the high-speed shaft 63 are not required.

[0046] In addition, the nacelle 4 typically includes a yaw mechanism 7 for yawing the nacelle 4 about the tower 2, particularly the vertical axis I. The yaw mechanism 7 may include a plurality of yaw motors 71 attached to the base of the nacelle 4 and includes a gear mechanism for reducing the rotational speed toward an output drive pinion that meshes with a toothed gear rim 72 on its inner side, which in turn connects to the top of the tower 2. Furthermore, a pitch mechanism 8 (at least partially) is included in the hub 54, wherein the pitch mechanism 8 is arranged to pitch the blades 51, 52, 53. In the current embodiment of the turbine 1, the pitch mechanism 8 includes three pitch drivers 81, 82, 83 arranged to drive the ends of the blade roots 55, 56, 57 of the respective blades 51, 52, 53. The pitch drivers 81, 82, 83 are arranged to individually drive the pitch rotation of the respective blades 51, 52, 53, such that the blades 51, 52, 53 can have different pitch angles at any given time. This pitch mechanism 8 is also called an independent pitch mechanism, and controlling the independent pitch mechanism to minimize fatigue loads on the turbine is called independent pitch control (IPC).

[0047] Figure 3An array of two wind turbines is schematically shown, where the wind direction W positions the second wind turbine 102 downstream of the wake of the first wind turbine 101. The wake can be considered as a region of reduced (average) wind speed with increased turbulence, as shown between dashed lines 103 and 104. The wake generated by wind turbine 101 will slowly mix with the surrounding (unaffected) wind field, and due to this mixing, the wake effect will decrease with increasing distance from the turbine. Turbines 101 and 102 are typically placed at a mutual distance d of three to ten times the rotor diameter (3D-10D), where a mutual distance of ten times the rotor diameter will significantly result in a lower wake effect (e.g., reduced power output and reduced vibration) compared to a mutual distance of only three times the rotor diameter, resulting in reduced induced fatigue loads on different wind turbine components. However, as mentioned above, wind farms typically must be developed within limited space, making longer mutual distances potentially lead to reduced power output across the entire farm, resulting in higher costs for the generated energy. Therefore, it is beneficial to increase wake mixing and reduce wake length and / or intensity, allowing the turbine to be placed at a smaller distance while still providing higher power output with less induced fatigue load.

[0048] Figure 4 A block diagram or flowchart 200 illustrates the steps of an embodiment of a control method for controlling a wind turbine. In step 201, periodic tilt and yaw functions 2011, 2012 are defined, wherein the periodic tilt and yaw functions 2011, 2012 are defined as sinusoidal functions having a common predetermined frequency f, and wherein the periodic tilt and yaw functions 2011, 2012 have a phase shift of preferably 90° or 270°. Thus, in this specific embodiment, referred to as the helical IPC, both tilt and yaw degrees of freedom are excited, but the phase shift is π / 2 rad (90°). This results in a time-varying torque on the rotor disk (as seen in a non-rotating coordinate system), completing one rotation every T = 1 / f seconds, and resulting in a helical wake 92, as in Figure 7 What I saw in the video.

[0049] The predetermined frequency f of the periodic tilt and yaw functions in 2011 and 2012 can be relative to the inflow wind speed U. ∞ And determined based on the turbine rotor diameter D, a dimensionless number known as the Struhal number:

[0050]

[0051] The optimal Struhal number is preferably between 0.05 and 1.0, more preferably between 0.15 and 0.55, even more preferably between 0.2 and 0.3, and most preferably around 0.25. This optimal value was estimated by performing a grid search under laminar conditions at different frequencies in a simulation program (Simulator for Wind Farm Application, SOWFA). The obtained average wake velocities at different distances after the excited turbine are... Figure 5 The diagram shows a graph representing the average wake velocity at different distances following a turbine excited at different frequencies. When greedy control is applied, the velocity is normalized by dividing by the wake velocity at each location. Figure 5 As shown, for several different distances from the rotor (where the distance is given in terms of several rotor diameters D (3D, 5D, and 7D)), the peak value is around St = 0.25. The Struhal number can be selected based on these results to determine the excitation frequency. Furthermore, the pitch amplitude β is preferably 15° or less, more preferably 10° or less, even more preferably 5° or less, and most preferably between 2° and 4°, because a (preferred) sinusoidal pitch variation with too large a pitch amplitude will result in an increase in load on the turbine.

[0052] The inverse multiblade coordinate (MBC) transformation step (203) is applied to obtain the periodic variations of the pitch angles θ1, θ2, and θ3 for the corresponding blades 51, 52, and 53. The MBC transformation decouples the blade loads in a non-rotating reference coordinate system, or in other words, projects the blade loads onto a non-rotating reference coordinate system, and is used in, for example, pitch control methods aimed at reducing fatigue loads on wind turbines. The rotor speed-dependent nth (nP) load harmonics per revolution are converted into steady-state contributions, thus simplifying controller design. Equations for implementing the transformation are summarized. The measured out-of-plane blade root bending moment M(t) ∈ R... B This is provided for a positive transformation, thereby transforming the rotating blade torque into a non-rotating reference coordinate system (e.g., as shown in step 207):

[0053]

[0054] in

[0055]

[0056] in, It is the harmonic order, B∈Z + It is the total number of leaves, and... It is a leaf The azimuth angle, where ψ = 0° indicates a vertical position. The common mode M0 represents the accumulated out-of-plane rotor torque, M... t and M y These represent a fixed coordinate system and tilting and yaw moments independent of azimuth (2071, 2072), respectively. The latter two components are typically used for fatigue load reduction purposes.

[0057] By applying the inverse MBC transformation to the non-rotating signal (in step 201), a separate pitch contribution can be achieved in the rotating (i.e., blade) coordinate system.

[0058]

[0059] in

[0060]

[0061] Where, θ 0,n θ t,n and θ y,n These are the common, tilt, and yaw pitch signals in the fixed coordinate system, respectively, and ψo,n is the azimuth offset of each harmonic.

[0062] Now, it is possible to use, for example, pitch drives 81, 82, 83 to individually drive the rotor blades to improve wake recovery, or in other words, increase wake mixing. By individually pitching the blades, the turbine thrust and subsequent power generation can be controlled to near greedy optimization (step 205).

[0063] As an example, the proposed control strategy is evaluated in the Wind Farm Application Simulator (SOWFA), a high-fidelity simulation environment developed by the National Renewable Energy Laboratory (NREL). SOWFA is a large eddy current solver for the hydrodynamics of turbulent atmospheres and their interaction with one or more wind turbines, taking into account Coriolis forces and buoyancy effects. The turbines are modeled as actuator disks or actuator lines. In this work, SOWFA is adapted to allow different pitch setpoints to be specified for individual blades. The simulation in this work is a neutral atmospheric boundary layer (ABL), where inflows are generated through so-called precursor simulations. Several characteristics of the simulation settings are listed below.

[0064] Numerical simulation schemes in SOWFA:

[0065]

[0066] As a baseline case for the control method according to the invention, a so-called greedy control strategy will be used. This method implies ignoring the interactions between wind turbines, thus all turbines operate at their respective optimal conditions. This implies that the rotor yaws perpendicular to the wind, and for below-rated wind conditions, the pitch angle and generator torque are controlled to achieve optimal power extraction from the wind. This case is used as a good baseline because it remains a commonly implemented strategy in wind farms. With this strategy, power generation from the upstream turbines is optimal, but the wake is relatively high, resulting in lower performance for the downstream machines.

[0067] In the control method according to an embodiment of the invention, the blades are individually pitched based on the periodic variations of the pitch angles θ1, θ2, θ3 to stimulate wake mixing by individually changing the induction factor of the blades and thus the yaw angle of attack of the turbine. As seen in step 207, this control method can apply yaw and tilt moments on the rotor by applying the MBC transformation, as described above. These yaw and tilt moments 207 can then lead to forced wake mixing, where the changes in power and wake velocity are small. This is achieved by superimposing the periodic variations of the pitch angles θ1, θ2, θ3 (step 204) on the common blade pitch angle of the wind turbine.

[0068] The projected load signals are first transformed into a rotating coordinate system using the aforementioned MBC transformation to obtain the achieved pitch angle. For the same sinusoidal roll and yaw signals (where the yaw signal has a 90° phase delay), this results in sinusoidal pitch signals β with different frequencies, according to commonly used trigonometric formulas:

[0069] β b =cos(ψ b (t))cos(2πft)+sin(ψ b (t))sin(2πft)

[0070] =cos(ψ b (t)+2πft)

[0071] =cos(2πf) h t+φ b ),

[0072] Where ψ b It is the azimuth position of blade b, f h It is a new spiral excitation frequency. This is the phase shift of blade b. Therefore, f can be determined. h =f+f r , where f r This is the rotor's rotational frequency. For the NREL 5MW reference turbine, in U... ∞The rotor speed of 8 m / s is equal to f r ≈9.5 rpm ≈0.158 Hz. Therefore, the pitch frequency f of the embodiment known as the spiral IPC is... h This will be slightly higher than the rotational frequency of the blades; for St = 0.25, f h ≈0.174Hz.

[0073] For example, alternative embodiments are found if the periodic tilt function 2011 is set to zero (yaw IPC), or alternatively, the periodic yaw function 2012 is set to zero (tilt IPC). In this case, the inverse MBC (step 203) and the commonly used trigonometric formula result in periodic changes in pitch angles θ1, θ2, θ3, which then become two sinusoidal signals (with a first frequency f). h =f+f r The first sinusoidal signal and having a second frequency f h =f r The superposition of the second sine signal (-f).

[0074] For example, in Figure 6A and Figure 6B The diagram schematically illustrates the effect of applying periodically varying pitch angles θ1, θ2, and θ3 to the corresponding blades 51, 52, and 53. Figure 6A The diagram illustrates a schematic representation of the position of the wake 9 (i.e., the position of the center of the wake, see the cross-section of the wake) at different times during a period T = 1 / f for an embodiment referred to as the tilting IPC. In the tilting IPC embodiment, the periodic yaw function 2012 is set to zero, and a sinusoidal tilting function with a predetermined frequency f is used to determine the predetermined periodic function used to change the pitch angle of the individual blades. The position of the wake 9 dynamically changes between an upper position and a lower position relative to the rotor 5 (as seen perpendicularly to the rotor plane IV) during the period T, where T = 1 / f.

[0075] exist Figure 6B The diagram illustrates the final change in the position of wake 9 due to the helical IPC. It shows the position of wake 9 relative to the rotor (i.e., the position of the wake center, as seen in the cross-section of the wake) at different moments during a period T = 1 / f. The position of wake 9 changes dynamically, as seen perpendicularly to the rotor plane IV. Wake 9 circles from its upper position relative to rotor 5 at t = 0, to its rightmost position at t = T / 4, to its lower position at t = T / 2, to its leftmost position at t = 3T / 4, and back to its upper position at t = T, thus completing one revolution over the period T, where T = 1 / f. Figure 7As shown and described below, the helical IPC embodiment results in a helical wake around the rotor axis, hence its name. These simulations have been performed with a 90° phase shift between the (same) tilt and yaw signals, resulting in a clockwise (CW) helical motion of the helix around the rotor axis, as seen from an upwind direction parallel to the rotor axis. With a 270° phase shift between the tilt and yaw signals, a counterclockwise (CCW) helical motion of the helix around the rotor axis is obtained.

[0076] Furthermore, wakes can be generated not only at different locations, but also in different directions, either separately or alternatively.

[0077] The effects of helical, tilt, and yaw IPC embodiments on turbine power generation have been evaluated, and a wake simulation of less than 1000 s has been performed as a result in the SOWFA solver. The results of these simulations are shown in the table below.

[0078]

[0079] This table presents simulation results for clockwise and counterclockwise helical IPC, yaw IPC, and roll IPC embodiments of the control method in SOWFA, where the clockwise (CW) and counterclockwise (CCW) helical IPC embodiments have been evaluated with pitch amplitudes β of 2.5° and 4°, respectively. Results are given in terms of power generation, power and thrust variations, and wake recovery. All results are shown relative to a baseline case with greedy control.

[0080] The new control method resulted in only 5.3% of the maximum power loss in the tested embodiments compared to the baseline. On the other hand, the amount of energy in the wake increased by up to 26%. The new control method also resulted in a reduction in power variation for all tested embodiments, meaning more constant power generation, which is beneficial for grid stability. In addition, power variation and thrust variation were also reduced, demonstrating that the method is advantageous not only in wind farm settings but also for individual turbines designed to provide more stable power output (i.e., smaller variations) and reduce some fatigue loads due to thrust (i.e., smaller thrust variations).

[0081] Figure 7On the left is shown a wind turbine 1, which includes a rotor 5 comprising three blades 51, 52, and 53, controlled by a greedy baseline condition. Downstream of turbine 1, a wake 91 is shown, where the gray portion suggests a reduced wind speed relative to the surrounding air. The darker the gray hue, the greater the reduction. Thus, wake 91 shows very little mixing even at a distance of ten times the rotor diameter (10D). Thrust 54, mirror-image relative to rotor plane IV, is also shown. The orientation of thrust 54 remains substantially stationary over time (i.e., it shows virtually no change in direction over time). Note that to clearly illustrate the resulting helical wake, a uniform inflow was used to obtain it. Figure 7 The simulation.

[0082] Figure 7 The right side shows the same turbine 1 controlled using a helical IPC embodiment of the control method, which generates a (rotating) helical wake 92 behind the turbine. Due to this helical wake 92, the mixing of the wake with the surrounding air is increased, thereby the wake dissolves more quickly. At a distance of approximately 5D, the wake effect has been significantly reduced. Thus, wind turbines controlled using the control method according to the invention can be placed closer to each other compared to more conventionally controlled wind turbines, thereby increasing the potential power generation of the wind farm. Thrust 55, mirror-image relative to the rotor plane IV, is also shown. Using the helical IPC embodiment, thrust 55 actually shows a slight change in orientation relative to the incoming wind when compared to thrust 54. It can be seen that during operation, the orientation of thrust 55 changes, and actually circulates along the rotor at the same pace as the initial portion (i.e., the origin) 93 of the wake 92. This involves the fact that a predetermined periodic function is defined such that the rotational position of the blades in the rotor plane changes from rotation to rotational displacement, at which the blades are at a first pitch angle.

[0083] As defined in this article, when referring to dynamically changing the position of the wake formed downstream of a wind turbine, the center of the wake (i.e., the geometric center of the wake in the cross-section, such as...) Figure 6A and Figure 6B As shown, an imaginary line relative to the rotor axis of the turbine (i.e. Figure 1A The location (i.e., the point) of line II in the diagram can actually be dynamically changed. Furthermore, when referring to dynamically changing the direction of the wake formed downstream of the wind turbine, the direction in which the wake is manipulated from the turbine can change dynamically.

[0084] The present invention is not limited to the embodiments shown, but extends to other embodiments that fall within the scope of the appended claims.

Claims

1. A method for controlling a wind turbine, the wind turbine including a rotor, the rotor including at least a first blade, the method comprising: While the first blade rotates, the induction factor of the first blade is changed over time by dynamically altering the blade pitch angle between the first and second pitch angles. Specifically, the dynamic change of the blade pitch angle over time is controlled such that the corresponding rotational position of the first blade at the first blade pitch angle in the rotor plane shifts with each rotation, and the corresponding rotational position of the first blade at the second blade pitch angle also shifts with each rotation. The induction factor of the first blade is changed over time at different angular positions in the rotor plane, such that the position and / or direction of the wake formed downstream of the wind turbine dynamically changes relative to the rotor of the wind turbine.

2. The method for controlling a wind turbine according to claim 1, wherein, The wake formed downstream of the wind turbine is a spiral wake.

3. The method for controlling a wind turbine according to claim 1, the method comprising: A yaw moment and / or tilting moment are applied to the rotor to achieve forced wake mixing by dynamically changing the pitch angle of the first blade of the wind turbine through periodic variations of the pitch angle superimposed on the pitch angle of the first blade.

4. The method for controlling a wind turbine according to claim 3, comprising: A yaw moment and a tilting moment are simultaneously applied to the rotor, wherein the yaw moment and the tilting moment have a common predetermined frequency and a predetermined phase offset relative to each other.

5. The method for controlling a wind turbine according to claim 1, wherein, The method of changing the induction factor of the first blade over time includes: dynamically changing the pitch angle of the first blade according to a predetermined periodic function, wherein the predetermined periodic function is defined as causing the blade to shift in a timely manner at the corresponding rotational position of the first blade at the first pitch angle and the second pitch angle.

6. The method for controlling a wind turbine according to claim 1, wherein, The dynamic change of the pitch angle of the first blade over time causes the orientation of the thrust experienced by the entire rotor to change periodically without changing the magnitude of the force itself.

7. The method for controlling a wind turbine according to claim 1, wherein, The rotor includes a second blade, and the method includes: changing the induction factor of the second blade over time by dynamically changing the pitch angle of the second blade between a first pitch angle and a second pitch angle, wherein the time the first blade is at the first pitch angle is different from the time the second blade is at the first pitch angle.

8. The method for controlling a wind turbine according to claim 5, wherein, The rotor includes a second blade, and the method includes: changing the induction factor of the second blade over time by dynamically changing the pitch angle of the second blade according to a predetermined periodic function, wherein the dynamic change of the pitch angle of the second blade differs from the dynamic change of the pitch angle of the first blade by a certain phase offset.

9. The method for controlling a wind turbine according to claim 8, wherein, The phase offset is equal to the cross angle between the first blade and the second blade in the rotor plane.

10. The method for controlling a wind turbine according to claim 1, wherein, The dynamic change of the pitch angle over time is obtained by performing an inverse multi-blade coordinate (MBC) transformation on a time-varying yaw function defined in a non-rotating reference coordinate system, or on a time-varying tilt function defined in a non-rotating reference coordinate system, or on a combination of the time-varying yaw function and the tilt function.

11. The method for controlling a wind turbine according to claim 10, wherein, The time-varying yaw function is a periodic yaw function and / or the time-varying tilt function is a periodic tilt function, wherein a predetermined periodic function is obtained by performing the inverse multi-blade coordinate (MBC) transformation, such that dynamically changing the pitch angle over time is dynamically changing the pitch angle according to the predetermined periodic function.

12. The method for controlling a wind turbine according to claim 11, wherein, The periodic tilt function and / or periodic yaw function are sinusoidal functions with a predetermined frequency.

13. The method for controlling a wind turbine according to claim 5, wherein, The predetermined periodic function includes a first sine function having a first frequency, wherein the first frequency is different from the rotational frequency of the rotor or a multiple thereof.

14. The method for controlling a wind turbine according to claim 13, wherein, The predetermined periodic function includes the superposition of the first sine function and the second sine function having a second frequency, wherein the first frequency and the second frequency are different.

15. The method for controlling a wind turbine according to claim 14, wherein, The value of the first frequency or the value of the second frequency is equal to the rotational frequency of the rotor as it increases or decreases with a predetermined frequency, wherein the predetermined frequency is a non-zero frequency less than the rotational frequency.

16. The method for controlling a wind turbine according to claim 15, wherein, The predetermined frequency is determined based at least on the diameter of the rotor, the rotational speed of the rotor, and / or the inflow wind speed determined upstream of the wind turbine.

17. The method for controlling a wind turbine according to claim 12, wherein, The predetermined frequency is determined at least based on the Struhal number, wherein the Struhal number is between 0.05 and 1.

0.

18. The method for controlling a wind turbine according to claim 1, wherein, The difference between the first pitch angle and the second pitch angle is 30° or less.

19. A wind turbine controller arranged for controlling a wind turbine, the wind turbine including a rotor, the rotor including at least a first blade, in, The controller is configured to change the induction factor of the first blade over time by dynamically changing the pitch angle of the first blade between a first pitch angle and a second pitch angle while the first blade is rotating. The dynamic change of the blade pitch angle over time is controlled such that the corresponding rotational position of the blade at the first pitch angle in the rotor plane shifts with each rotation, and the corresponding rotational position of the first blade at the second pitch angle shifts with each rotation in the rotor plane. The induction factor of the first blade is changed over time at different angular positions in the rotor plane, such that the position and / or direction of the wake formed downstream of the wind turbine dynamically changes relative to the rotor of the wind turbine.

20. The wind turbine controller of claim 19, configured to perform the method of any one of claims 1 to 18.

21. A wind turbine, comprising a rotor, the rotor including at least a first blade, the wind turbine further comprising a wind turbine controller according to claim 19.

22. An array of at least two wind turbines, wherein, For a given wind direction, the second wind turbine is at least partially arranged downstream of the wake of the first wind turbine, wherein the first and second wind turbines include rotors, the rotors including at least a first blade, and wherein at least the first wind turbine includes a wind turbine controller according to claim 19.

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