Enhanced wind turbine wake mixing

Abstract

1. A method of controlling a wind turbine including a rotor including at least a first blade, the method comprising: varying an induction coefficient of the first blade over time by dynamically varying a pitch angle of the first blade over time between a first pitch angle and a second pitch angle while the first blade is rotating, the first pitch angle and the second pitch angle being different, the dynamic variation of the pitch angle of the first blade over time displacing in time a rotational position of each of the first blades at the first pitch angle and a rotational position of each of the first blades at the second pitch angle of the first blade, the variation of the induction coefficient of the blade being controlled to occur at different angular positions in a rotor plane over time, whereby a position and / or direction of a wake formed downstream of the wind turbine dynamically changes relative to the rotor of the wind turbine.

Description

[Technical field]

[0001] The present disclosure relates to a method for controlling a wind turbine, a wind turbine controller arranged for the method of controlling a wind turbine, and a wind turbine including a wind turbine controller arranged for the method of controlling a wind turbine and an array of wind turbines, where at least a first wind turbine includes a wind turbine controller. [Background technology]

[0002] To meet the 1.5°C climate stabilization target set by the Paris Agreement, fossil fuel electricity production will need to be significantly reduced in the coming years. To replace this capacity, a large increase in renewable energy sources such as solar and wind energy will be required.

[0003] The most effective approach to developing wind energy on a large scale is to deploy individual wind turbines either onshore or offshore, in so-called wind farms. These wind farms consist of multiple wind turbines (usually of the same type) divided into a specific area and usually sharing a common infrastructure, thus limiting the use of land and / or sea space while reducing the overall capital operating costs of the turbines and allowing the wind farm to be efficiently maintained. Summary of the Invention [Problem to be solved by the invention]

[0004] However, as wind turbines extract energy from the wind, they generate a wake downstream of the turbine, where the (average) wind speed decreases and the (average) turbulence increases. This has a negative impact on turbines located further downstream, because reduced wind speed reduces energy production and increased turbulence increases fatigue loads on downstream turbines.

[0005] As the flow travels further downstream, the wake spreads out, thereby mixing with the surrounding free-stream wind, causing the wake to recover over time (and therefore distance) towards free-stream conditions.Due to the fact that wind farm placement is at least a trade-off between available area, installed power, and infrastructure costs, it is economically highly unfavourable to place wind turbines at such a mutual distance that wake effects are minimized.

[0006] To optimize power output at wind farm level, rather than at individual wind turbine level, wind farm control studies focused on steady-state optimal control, also known as axial guidance control or derating approach. There, wind turbines located upstream of a wind farm are controlled to lower their power output (i.e. reduce their performance), thereby allowing wind turbines located downwind to extract a little more energy from the wind passing them. The goal was to find optimal static control parameters, regardless of wind and turbine dynamics. However, it was found that the potential power gains of such static approaches, compared to a steady-state "greedy" strategy where all turbines operate at their individual optima, could even lead to a reduction in overall power output.

[0007] It may be a goal of the present disclosure, possibly among other goals, to obtain a method for controlling a wind turbine that reduces wake effects downstream of the wind turbine, in which at least one of the above problems is at least partially alleviated. [Means for solving the problem]

[0008] This objective may be achieved, among other objectives, at least in part by a method of controlling a wind turbine including a rotor including at least a first blade, the method comprising the step of varying an induction coefficient (particularly a radial induction coefficient) of the first blade over time by dynamically varying a pitch angle of the first blade over time between a first pitch angle and a second pitch angle while the first blade is rotating, the first pitch angle and the second pitch angle being different, and the dynamic variation of the pitch angle over time displaces respective rotational positions of the first blade at the first and second pitch angles of the blade over time, such that a position and / or direction of a wake formed downstream of the wind turbine dynamically changes relative to the rotor of the wind turbine.

[0009] In a further aspect of the disclosure, 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, it is also possible to control the dynamic change of the pitch angle over time such that the shape (but not 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 can, for example, correspond to a wake having a substantially constant shape rotating about the axis of the rotor over time (preferably at a rotational speed (significantly) lower than the rotational speed of the rotor).

[0010] A wind turbine includes a rotor including at least one blade (and typically multiple blades, e.g., two or three blades) to convert the kinetic energy of the wind into mechanical energy that is converted into electrical energy by a generator.

[0011] Specifically, wind speed causes the blades to rotate, which in turn provides power to the generator. However, the rotating blades (effectively forming a rotor disk) slow the wind, forming a wake behind the turbine. The wind in the wake has a lower average speed and higher average turbulence than wind that passes around the rotor disk without passing through the turbine's blades. Furthermore, the slower wind speed in the wake compared to the wind speed unaffected by the rotor causes the wake to expand; that is, the diameter of the wake expands beyond the diameter of the rotor. In general, the wake defines the volume of fluid (e.g., air) that is affected by the turbine's blades. A turbine located downstream (downwind) in this wake can only use a relatively slower wind speed to rotate its blades, thereby reducing the power output of the downwind turbine. Furthermore, due to the increased mean turbulence, the downwind turbine also experiences more fatigue loads, thereby negatively affecting its lifespan.

[0012] By transferring kinetic energy from the wind surrounding the wake to the wake itself, the difference in the relative wind speed and turbulence between the wake and the surrounding air slowly decreases over time. This process is called turbulent mixing. Because turbulent mixing occurs naturally, the kinetic energy transferred from the wind to the wind turbine is eventually transformed. However, the distance required to transfer the kinetic energy to the wake varies with the wind speed. Thus, the distance between a first and a second turbine placed close to each other (typically less than 10 times the rotor diameter, i.e. 10D) may not be enough to return the kinetic energy taken by the first turbine before the wind reaches the second turbine placed downstream of the first wind turbine.

[0013] In general, the induction coefficient is determined by the wind speed V upstream of the rotor plane. ∞ and the wind speed V at the rotor plane (i.e., the rotor disk). d The difference between the upstream wind speed V ∞ It is determined by dividing by:

number

[0014] Also, by displacing in time the respective rotational positions of the first blade at the first and preferably second pitch angles of the blade, the induction variation (i.e. the varying induction coefficient) of the blade occurs at different angular positions in the rotor plane over time. This also causes the position of the wake formed downstream of the wind turbine to dynamically change relative to the rotor of the wind turbine. Such a change in the position of the wake increases the turbulent mixing, resulting in a reduction in the distance required to transfer kinetic energy to the wake, which may be placed downstream of the wind turbine, thereby being much less affected by the wake. In the case of multiple blades, the pitch angle of at least the first blade is preferably arranged to be changed individually relative to the other blades.

[0015] In an optional embodiment, the method includes adding a yaw and / or tilt motion to the rotor to obtain forced wake mixing by superimposing a periodic change in pitch angle on the pitch angle of the first (e.g., collective) blades of the wind turbine so as to dynamically vary the pitch angle of the first blades. Thus, an advantage of this approach is that improved wake mixing can be obtained by applying only minor modifications to existing control approaches of wind turbines (i.e., by superimposing a periodic change on the pitch angle).

[0016] In an optional embodiment of the method, varying the induction coefficient of the first blade over time further comprises dynamically varying a pitch angle of the first blade according to a predefined periodic function, the predefined periodic function being defined such that the respective rotational positions of the first blade at the first and second blade pitch angles vary in time. A periodic variation, i.e. according to a predefined periodic function, is a simple and effective way to ensure that the respective rotational positions of the first blade at the first and second blade pitch angles vary in time, thereby allowing to dynamically change the position and / or direction of the wake formed downstream of the wind turbine.

[0017] In an optional embodiment of the method, the predefined periodic function is defined such that the rotational position of the blade in the rotor plane at the first pitch angle of the blade is displaced with each revolution. Due to the induced fluctuations of the blade occurring at various angular positions in the rotor plane, the resulting thrust acting on the blade also changes with the induced fluctuations, which causes a periodic change in the direction of the thrust to be experienced by the entire rotor without significantly changing the amplitude of the force itself. Therefore, only relatively small fluctuations of the thrust are experienced, so that they do not add much to the fatigue load induced in the turbine. In particular, if the displacement with each revolution is relatively slow, for example less than 180° per revolution, preferably less than 90° per revolution, more preferably less than 45° per revolution, then periodically changing the direction of the thrust will result in less frequent force fluctuations and will not lead to a significant increase in the fatigue load of the turbine.

[0018] In an optional embodiment, the rotor includes a second blade, and the method includes varying an induction coefficient of the second blade over time by dynamically varying a pitch angle of the second blade between a first pitch angle and a second pitch angle, wherein a time that the first blade is at the first pitch angle is different from a time that the second blade is at the first pitch angle. Varying the pitch angle of the first blade may introduce an imbalance into the rotor. As described herein, the imbalance may be at least partially eliminated by dynamically varying the pitch angle of the second blade as well.

[0019] In an optional embodiment, the rotor includes a second blade, preferably arranged such that the pitch angle of the second blade can be individually changed relative to or independent of the pitch angle of the first blade, and the method preferably includes varying the induction coefficient of the second blade over time by dynamically varying the pitch angle of the second blade according to a predefined periodic function, the dynamic change in the pitch angle of the second blade differing from the dynamic change in the pitch angle of the first blade by a phase offset. Also, by varying the induction coefficient of the second blade according to a predefined periodic function by a phase offset relative to the first blade, the pitch angles of the first and second blades (and therefore their induction coefficients) are not simultaneously at a maximum or minimum. If the induction coefficients were to be simultaneously at a maximum or minimum, the induction coefficient of the rotor as a whole would change, effectively derating the turbine and reducing electrical production, rather than creating local variations that only have a small effect on the induction coefficient of the rotor as a whole and therefore on the power output of the turbine.

[0020] The phase offset can be selected to be substantially equal to the intersection angle between the first and second blades in the rotor plane. For example, for a two-bladed turbine, the intersection angle is approximately 180°. With a phase offset of approximately 180°, the minimum induction coefficient of the first blade is simultaneously compensated by the maximum of the second blade, which results in a minimal variation of the induction coefficient across the rotor. For example, for a three-bladed turbine, the intersection angle is approximately 120°. With a phase offset of approximately 120°, the minimum induction coefficient of the first blade is substantially compensated by the other two blades.

[0021] According to an optional embodiment of the method, the dynamic change of the pitch angle over time is obtained by performing an inverse multiblade coordination (MBC) transformation on a time-varying yaw function defined in a non-rotating reference frame, or a time-varying tilt function defined in a non-rotating reference frame, or a combination of time-varying yaw and tilt functions. Multiblade coordination (MBC) is typically used to transform blade moments from a local blade frame to a non-rotating or ground-fixed inertial frame to determine loads on a turbine tower, for example. By defining the time-varying yaw and / or tilt signals in a non-rotating frame, each capable of turning the wake horizontally and / or vertically, and performing an inverse MBC transformation, the signals are transformed to the local blade frame, thereby obtaining the dynamic change of the pitch angle of at least a first blade.

[0022] In an embodiment of the present disclosure, the time-varying yaw function is a periodic yaw function and / or the time-varying ramp function is a periodic ramp function. By performing the inverse multi-blade coordinate (MBC) transformation, a predefined periodic function is obtained, whereby dynamically varying the pitch angle over time results in dynamically varying the pitch angle according to the predefined periodic function. This allows for the implementation of a relatively simple function to achieve the desired effect described above. Preferably, the periodic ramp function and / or the periodic yaw function are sinusoidal functions with a predefined frequency. This allows for the inverse transformation to obtain periodic pitch functions of the individual blades, whereby the periodic pitch functions are also sinusoidal functions, or a superposition of sinusoidal functions, resulting in a smooth pitch signal. A smooth pitch signal is preferred, especially due to the size and weight of the individual blades of utility-scale wind turbines. This ensures that no excitations like sudden shocks are introduced through the pitch mechanism. These excite any kind of unwanted dynamics in the wind turbine structure, resulting in increased loads on the turbine and its components.

[0023] In one embodiment of this method, the predefined periodic function comprises a first sinusoidal function having a first frequency, the first frequency being different from a rotational frequency of the rotor or a multiple thereof. Alternatively, the predefined periodic function comprises a superposition of the first sinusoidal function and a second sinusoidal function having a second frequency, the first and second frequencies being different. As explained above, a sinusoidal function or a superposition of sinusoidal functions provides a smooth periodic variation of the pitch angle.

[0024] The first frequency value or the second frequency value can be selected to be substantially equal to the rotational frequency of the rotor increased or decreased by a predetermined frequency, the predetermined frequency being a non-zero frequency less than the rotational frequency. Thus, the dynamic change of the pitch angle of at least the first blade according to the predefined periodic function is a relatively slow change compared to the rotational frequency of the turbine. These low frequency signals therefore result in slow and smooth periodic pitch angle changes that are not expected to result in a significant increase in the load on the turbine, while at the same time, a relatively slow change in the position and direction of the wake is obtained, resulting in increased wake mixing. Since the predetermined frequency is a non-zero frequency less than the rotational frequency, the above-mentioned improved wake mixing effect is obtained while the pitching action of the blades is only slightly increased, when compared to existing separate pitch control methods for load reduction, for example, where additional load on the pitch system, especially the pitch bearing, which is often the most fatigue-critical part of the pitch system, is limited.

[0025] The predefined frequency is preferably determined depending on at least the diameter of the rotor, the rotational speed of the rotor, and / or the inlet wind speed determined upstream of the wind turbine, whereby a predefined periodic function can be customized for different operating conditions or turbine sizes to increase wake mixing for different operating conditions and turbine sizes. Alternatively or additionally, the predefined frequency is preferably determined depending on at least the Strouhal number, the Strouhal number being preferably between 0.05 and 1.0, more preferably between 0.15 and 0.55, even more preferably between 0.2 and 0.3, most preferably about 0.25.

[0026] The predetermined frequency of the periodic tilt and / or yaw function can be determined by a dimensionless number called the Strouhal number:

number

[0027] In an optional embodiment of this method, the difference between the first and second pitch angles is 30° or less, preferably 20° or less, more preferably 10° or less, and most preferably 2°-8°. Too much pitch angle variation reduces turbine performance, while too little variation does not generate the required amount of wake mixing. A suitable tradeoff between the two has been found in the above range.

[0028] In a second aspect of the disclosure, there is provided a wind turbine controller arranged to control a wind turbine including a rotor including at least a first blade, the controller being arranged to vary an induction coefficient of the first blade over time by dynamically varying a pitch angle of the first blade according to a predefined periodic function to periodically vary a pitch angle of the first blade between a first pitch angle and a second pitch angle while the first blade is rotating, the first pitch angle and the second pitch angle being different, the predefined periodic function being defined such that respective rotational positions of the first blade at the first pitch angle and the second pitch angle of the blade are displaced in time, whereby the controller is arranged to dynamically vary a position of a wake formed downstream of the wind turbine relative to the rotor of the wind turbine, whereby the advantages of the control method are applied to the controller.

[0029] In a third aspect of the present disclosure, there is provided a wind turbine including a rotor including at least a first blade, said wind turbine further including a wind turbine controller arranged for a method of controlling the wind turbine according to any of the given embodiments, resulting in a wind turbine capable of improving mixing in a wake formed downstream of the turbine.

[0030] In a further aspect, an array of at least two wind turbines is provided, where for a given wind direction, a second wind turbine is arranged at least partially downstream in a wake of a first wind turbine, where the first and second wind turbines comprise rotors including at least a first blade, and where at least the first wind turbine further comprises a wind turbine controller arranged for a method of controlling a wind turbine according to any of the given embodiments, whereby an array of turbines, e.g. a wind farm, is obtained, where at least one turbine is configured for improving mixing in a wake formed downstream of the turbine, such that the electrical energy production of the array of wind turbines, i.e. the wind farm, can be further increased.

[0031] The present disclosure is further illustrated by the following figures, which show exemplary embodiments of a method for controlling a wind turbine according to the present disclosure and are not intended to limit the scope of the present disclosure in any way: [Brief description of the drawings]

[0032] [Figure 1A] FIG. 1A shows a schematic of a horizontal axis wind turbine with a three-bladed rotor. [Figure 1B] FIG. 1B shows a schematic of an angled blade. [Diagram 2] FIG. 2 shows a schematic of a wind turbine nacelle and rotor made up of its various components. [Diagram 3]Figure 3 shows a schematic of an array of two wind turbines, the second of which is positioned downstream in the wake of the first. [Figure 4] FIG. 4 shows a flow chart including the various steps involved in an embodiment of a method for controlling a wind turbine. [Diagram 5] Figure 5 shows a graph depicting the average wake velocity at different distances behind the turbine controlled based on different Strouhal numbers. The velocities are normalized by dividing by the wake velocity at each location when greedy control is applied. [Figure 6A] FIG. 6A shows diagrammatically the position of the wake at different times during one excitation period T of a turbine controlled by two different method embodiments, respectively. [Figure 6B] FIG. 6B shows diagrammatically the position of the wake at different times during one excitation period T of a turbine controlled by two different method embodiments, respectively. [Figure 7] FIG. 7 illustrates the difference between the wake generated by a turbine controlled using a greedy control approach and a turbine controlled using an embodiment of the method for controlling a wind turbine according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] FIG. 1A shows diagrammatically the layout of a typical three-bladed horizontal axis wind turbine 1. The wind turbine comprises a tower 2 situated on top of a foundation 3. It is noted that such wind turbines can be located on land (e.g. onshore) and offshore (e.g. offshore). In the latter case, the foundation 3 is typically an offshore foundation, e.g. a subsea fixed structure installed on the seabed, such as a monopile, tripod, jacket or even a floating foundation, with a buoyant body anchored to the seabed to keep it in place. In the case of onshore turbines, such a foundation 3 is typically a so-called gravity foundation, comprising a heavy concrete body to keep the wind turbine 1 fixed to the ground.

[0034] The nacelle 4 coupled to the rotor 5 is arranged at the top of the tower 2. The rotor 5 includes three blades 51, 52, 53, although any number of blades is possible, for example one, two or four blades may also be applied. The blades 51, 52, 53 are fixed to a hub 54. The rotation of the nacelle 4 about a vertical axis I, which is substantially parallel or coincident with the tower 2 and substantially perpendicular to the ground, is called yaw rotation. The yaw angle can be defined depending on the wind direction, where a non-zero yaw angle means that there is a deviation 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 being often called azimuth rotation. The blades 51, 52, 53 are further arranged to rotate about their respective longitudinal axes III, this rotation being called pitch rotation, and the angle between the central axis V of the cross section of the blades 51, 52, 53 relative to the plane of rotation IV of the rotor 5 is called the pitch angle. FIG. 1B shows the central axis V of a cross section of a first blade 51 pitched at a pitch angle θ1 relative to a rotor plane IV.

[0035] Figure 2 shows diagrammatically the nacelle 4 and rotor 5 of a wind turbine 1, with the different components arranged in the nacelle 4. The nacelle 4 houses the drive train 6, which may comprise a generator 61 for generating electrical energy, 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, while the high-speed shaft 63 transmits the rotation from the output of the gearbox 62 to the generator 61. It is noted that in so-called direct drive wind turbines, the rotor is usually connected directly to the generator via the main shaft or the low-speed shaft. In these types of wind turbines, the gearbox 62 and the high-speed shaft 63 are not needed.

[0036] Furthermore, the nacelle 4 typically also includes a yaw mechanism 7 for yawing the nacelle 4 about the tower 2, in particular about the vertical axis I. The yaw mechanism 7 may comprise several yaw motors 71, including gears for reducing the rotational speed towards an output drive pinion, which is mounted to the base of the nacelle 4 and may mesh with an internally toothed gear rim 72, which is connected to the top of the tower 2. Furthermore, a pitch mechanism 8 is included (at least partially) within the hub 54, the pitch mechanism 8 being arranged to pitch the blades 51, 52, 53. In the present embodiment of the turbine 1, the pitch mechanism 8 includes three pitch drives 81, 82, 83 arranged to drive the ends of the blade root sections 55, 56, 57 of the respective blades 51, 52, 53. The pitch drives 81, 82, 83 are arranged to individually drive the pitch rotation of each blade 51, 52, 53, such that the blades 51, 52, 53 can all have different pitch angles at any given time. Such a pitch mechanism 8 is also called an individual pitch mechanism, and controlling the individual pitch mechanism to minimize fatigue load on the turbine is called Individual Pitch Control (IPC).

[0037] FIG. 3 shows a schematic representation of an array of two wind turbines, with a wind direction W such that a second wind turbine 102 is placed downstream in the wake of a 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, 104. The wake caused by the wind turbine 101 slowly mixes with the surrounding (unaffected) wind field, and due to this mixing, the wake effect decreases with increasing distance from the turbine. The turbines 101, 102 are typically placed at a mutual distance d of 3 to 10 rotor diameters (3D-10D), where a mutual distance of 10 rotor diameters clearly reduces wake-induced effects such as reduced power output and reduced vibrations, and thereby reduced fatigue loads on the various wind turbine components, than a mutual distance of only 3 times the distance. However, as already explained above, wind farms usually have to be developed within a limited space. Thus, increased mutual distances can result in lower overall farm power output and higher costs of generated energy. It is therefore beneficial to be able to increase wake mixing and reduce the length and / or strength of the wake, thereby allowing turbines to be placed at closer distances while at the same time providing higher power output with less induced fatigue loads.

[0038] FIG. 4 shows a block chart or flow chart 200 of 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, where the periodic tilt and yaw functions 2011, 2012 are defined as sine functions with a common predefined frequency f, and the periodic tilt and yaw functions 2011, 2012 have a specific phase offset, preferably 90° or 270°. Thus, in this particular embodiment, called helical IPC, both tilt and yaw degrees of freedom are excited, but with a phase offset of π / 2 rad (90°). This creates a moment on the rotor disk (as seen in a non-rotating frame) that rotates over time, completing one rotation every T=1 / f seconds, leading to a helical wake 92 as shown in FIG. 7.

[0039] A given frequency f of the periodic tilt and yaw functions 2011, 2012 is a function of the inlet wind speed U based on a dimensionless number called the Strouhal number. ∞ and for the turbine rotor diameter D it can be determined:

number

[0040] An inverse multi-blade coordinate (MBC) transformation step (203) is applied to obtain the periodic variation of the pitch angles θ1, θ2, θ3 of each blade 51, 52, 53. The MBC transformation decouples, or in other words projects, the blade loading in a non-rotating reference frame, which is the transformation used, for example, in individual pitch control approaches aimed at lowering fatigue loads in wind turbines. The rotor speed dependent n times per revolution (nP) load harmonics are transformed into steady-state contributions, simplifying the controller design. The equations that perform the transformation are summarized: Measured out-of-plane blade root bending moment M(t) ∈ R B is fed into a forward transformation to transform the rotating blade moment into a non-rotating reference frame (e.g., also shown in step 207):

number

number

[0041] By applying the inverse MBC transform to the unrotated signal (step 201), this results in individual pitch contributions that can be implemented in the rotated (ie blade) frame.

number

number

[0042] The possibility of individually driving the pitch of the rotor blades, for example using pitch drives 81, 82, 83, is currently employed to increase the wake recovery effect, in other words to increase wake mixing. By pitching the blades individually, the turbine thrust, and subsequently the power generation, can be controlled close to the greedy optimum (step 205).

[0043] The proposed control strategies are evaluated as an example in the Simulator for Wind Farm Applications (SOWFA), a high-fidelity simulation environment developed by the US National Renewable Energy Laboratory (NREL). SOWFA is a large-eddy solver for the fluid dynamics in a turbulent atmosphere and its interaction with one or more wind turbines, taking into account the effects of Coriolis forces and buoyancy. The turbines are modeled as actuator disks or actuator lines. In this work, SOWFA was tuned to allow different pitch settings for each individual blade. The simulation in this work is a simulation of a neutral atmospheric boundary layer (ABL) where the inflow is generated by a so-called precursor simulation. Some properties of the simulation setup are given below:

[0044] Numerical simulation scheme of SOWFA: Turbine: NREL 5MW reference turbine Rotor diameter: 126.4m Domain size: 3km x 3km x 1km Cell size (outer area): 10m x 10m x 10m Cell size (near rotor): 1.25m x 1.25m x 1.25m ABL Stability: Neutral Inflow wind speed: 8.0m / s Inflow turbulence intensity: 5.9%

[0045] As a reference line case for the control method according to the present disclosure, a so-called greedy control strategy is used. This approach shows that the interactions between the wind turbines are ignored and therefore all turbines operate at their individual optimum. This means that the rotors are yawed perpendicular to the wind and in below-rated wind conditions the pitch angle and generator torque are controlled to achieve optimal power extraction from the wind. This case serves as a suitable reference line since it is a strategy that is still commonly implemented in wind farms. In this strategy, the power generation of the upstream turbine is optimal, but the wake defects are relatively large, leading to poor performance of the downstream machines.

[0046] In a control method according to one embodiment of the present disclosure, pitching the blades individually according to a periodic change in pitch angles θ1, θ2, θ3 is used to stimulate wake mixing by individually changing the induction coefficient of the blades and thereby changing the yaw angle of attack of the turbine. The method of control allows for the addition of yaw and tilt motions to the rotor, as seen in step 207, by applying the MBC transformation, as explained above. These yaw and tilt motions 207 can then lead to forced wake mixing with small variations in power and wake velocity. This is achieved by superimposing a periodic change in pitch angles θ1, θ2, θ3 (step 204) on the collective blade pitch angles of the wind turbine.

[0047] These projected load signals are first transformed into a rotating frame using the MBC transformation described above to obtain the implemented pitch angle. For identical sinusoidal tilt and yaw signals, with a phase delay of 90° for the yaw signal, this leads to sinusoidal pitch signals β of different frequencies, according to the general trigonometric formula:

number

[0048] Alternative embodiments are found, for example, if the periodic tilt function 2011 is set to zero (yaw IPC) or instead the periodic yaw function 2012 is set to zero (tilt IPC). In that case, the pitch angles θ1, θ2, θ3 are periodically varied by inverse MBC (step 203) and common trigonometric formulas. In this case, the periodic variation becomes a superposition of two sinusoidal signals, with a first frequency f h =f+f r The first sinusoidal signal has a frequency of f h =f r -f has a second sine wave signal.

[0049] The effect of applying a periodic change in pitch angles θ1, θ2, θ3, for example, to each blade 51, 52, 53, is shown diagrammatically in Figures 6A and 6B. Figure 6A shows a schematic diagram of the position of the wake 9 (i.e., the position of the wake center as seen in the cross section of the wake) at various times during one period T = 1 / f for an embodiment called tilted IPC. In this tilted IPC embodiment, the periodic yaw function 2012 is set to zero and a sinusoidal ramp function with a predetermined frequency f is used to determine the predetermined periodic function with which the pitch angles of the individual blades change. The position of the wake 9 changes dynamically between an up and down position with respect to the rotor 5, as seen perpendicular to the rotor plane IV, over the course of a period T, where T = 1 / f.

[0050] FIG. 6B shows the resulting change in the position of the wake 9 with the helical IPC. The position of the wake 9 relative to the rotor (i.e., the position of the center of the wake as seen in a cross-section of the wake) at different time instances during one period T=1 / f. The position of the wake 9 is dynamically changing as seen perpendicular to the rotor plane IV. The wake 9 moves from an upper position relative to the rotor 5 at t=0 to a right-most position at t=T / 4, a lower position at t=T / 2, a left-most position at t=3T / 4, back to the top position at t=T, and circles around, thereby completing one revolution over the course of a period T, where T=1 / f. As shown in FIG. 7 and described below, the helical IPC embodiment results in a helical wake around the rotor axis, hence the name. These simulations were performed with a phase offset of 90° between the (identical) tilt and yaw signals, resulting in a clockwise (CW) screwing movement of the helix around the rotor axis when viewed from an upwind direction parallel to the rotor axis. A phase offset of 270° between the tilt and yaw signals results in a counter-clockwise (CCW) screwing movement of the helix around the rotor axis.

[0051] Furthermore, not only may the wakes be generated at different locations, but the wakes may additionally or alternatively be directed in different directions.

[0052] To evaluate the impact of the helix, tilt, and yaw IPC embodiments on the turbine power production, 1000s simulations of the resulting wake defects were performed in the SOWFA solver. The results of these simulations are shown in the following table.

[0053] [Table 1]

[0054] The table shows the results of simulations in SOWFA for clockwise and counterclockwise helical IPC, tilt IPC, and yaw IPC embodiments of the control strategy, where the clockwise (CW) and counterclockwise (CCW) helical IPC embodiments are evaluated at pitch amplitudes β of 2.5° and 4°. Results are shown in terms of power generation, power and thrust variations, and wake recovery. All results are shown for the greedy control baseline case.

[0055] The new control method results in a maximum power loss of only 5.3% compared to the baseline of the tested embodiments. Meanwhile, the amount of energy in the wake increases by up to 26%. The new control method also results in a reduction in the power fluctuations of all the tested embodiments, which means a more constant power production is obtained, which is beneficial for grid stability. Furthermore, the power fluctuations and thrust fluctuations are also reduced, indicating that this method is beneficial not only for wind farm installations, but also for individual turbines aiming to provide a more stable power output (i.e. less fluctuations) and reduce thrust fatigue loads (i.e. less thrust force fluctuations).

[0056] Figure 7 shows, on the left, a wind turbine 1 including a rotor 5 including three blades 51, 52, 53 controlled with a reference line case of greedy control. Downstream of the turbine 1, a wake 91 is shown, the grey sections signifying a reduction in wind speed with respect to the surrounding air. The darker the grey tones, the greater the reduction. It can thus be seen that the wake 91 shows almost no signs of mixing, even at a distance of ten rotor diameters (10D). Also shown is the thrust 54 mirrored with respect to the rotor plane IV. Over time, the direction of the thrust 54 remains substantially stationary (i.e., shows substantially no change in direction over time). It is noted that the simulations to obtain Figure 7 have been carried out based on a uniform inflow in order to clearly show the resulting spiral wake.

[0057] The right part of FIG. 7 shows that the same turbine 1 controlled using the helical IPC embodiment of the control method results in a (rotating) helical wake 92 behind the turbine. This helical wake 92 increases the wake mixing with the surrounding air, which makes the wake dissipate much faster. Already at a distance of about 5D, the wake effect is significantly reduced. Thereby, wind turbines controlled with the control method according to the present disclosure can be placed closer to each other compared to more traditionally controlled wind turbines, thereby increasing the potential power generation of the wind farm. Also shown is the thrust 55 mirrored with respect to the rotor plane IV. Using the helical IPC embodiment, the thrust 55 actually shows a slight change in orientation with respect to the incoming wind when compared to the thrust 54. It can be seen that the direction of the thrust 55 changes during operation and actually circulates along the rotor at the same pace as the circulation of the initial section (i.e., origin) 93 of the wake 92. This is related to the fact that a predefined periodic function is defined such that the rotational position of the blade in the rotor plane at the first pitch angle of the blade is displaced with each revolution.

[0058] As defined herein, when referring to dynamically changing the position of the wake formed downstream of a wind turbine, the position of the wake center (i.e., the geometric center of the wake in cross section, as shown in Figures 6A and 6B, which is relative to an imaginary line in the direction of the rotor axis of the turbine (i.e., line II in Figure 1A)) may actually change dynamically. Furthermore, when referring to dynamically changing the direction of the wake formed downstream of a wind turbine, the direction in which the wake is turned away from the turbine may change dynamically.

[0059] The present disclosure is not limited to the illustrated embodiments, but extends to other embodiments that fall within the scope of the appended claims.

Claims

1. 1. A method of controlling a wind turbine including a rotor including at least a first blade, comprising: dynamically varying a pitch angle of the first blade over time between a first pitch angle and a second pitch angle while the first blade is rotating, thereby varying an induction coefficient of the first blade over time; the first pitch angle and the second pitch angle are different; the dynamic change in pitch angle over time is controlled such that a rotational position of each of the first blades in a rotor plane at a first pitch angle of the first blades is displaced with each revolution, and a rotational position of each of the first blades at a second pitch angle of the first blades is displaced with each revolution, such that a change in induction coefficient of the blades occurs at different angular positions in the rotor plane over time, whereby a position and / or direction of a wake formed downstream of the wind turbine dynamically changes relative to the rotor of the wind turbine. A method for controlling a wind turbine.

2. 2. The method of controlling a wind turbine according to claim 1, comprising the step of applying a yaw and / or tilt motion to the rotor to obtain forced wake mixing by superimposing a periodic change in pitch angle on the pitch angle of the first blade of the wind turbine so as to dynamically change the pitch angle of the first blade.

3. 3. The method of controlling a wind turbine according to claim 1 or 2, wherein varying the induction coefficient of the first blade over time comprises dynamically varying a pitch angle of the first blade according to a predefined periodic function, the predefined periodic function being defined such that respective rotational positions of the first blade at the first and second pitch angles of the blade vary in time.

4. 4. The method of controlling a wind turbine according to claim 1, wherein the rotor includes a second blade, the method comprising the step of varying an induction coefficient of the second blade over time by dynamically varying a pitch angle of the second blade between a first pitch angle and a second pitch angle, the time that the first blade is at the first pitch angle being different from the time that the second blade is at the first pitch angle.

5. 5. The method of controlling a wind turbine according to claim 3 or 4, wherein the rotor includes a second blade, the method comprising varying an induction coefficient of the second blade over time by dynamically varying a pitch angle of the second blade according to a predefined periodic function, the dynamic change in pitch angle of the second blade differing from the dynamic change in pitch angle of the first blade by a phase offset.

6. 6. The method of controlling a wind turbine according to claim 5, wherein the phase offset is substantially equal to an angle of intersection between the first blade and the second blade in a rotor plane.

7. 7. A method of controlling a wind turbine according to any one of claims 1 to 6, wherein the dynamic change in 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 frame, or a time-varying ramp function defined in a non-rotating reference frame, or a combination of the time-varying yaw function and ramp function.

8. 8. The method of controlling a wind turbine of claim 7, wherein the time-varying yaw function is a periodic yaw function and / or the time-varying ramp function is a periodic ramp function, and performing the inverse multi-blade coordinate (MBC) transformation results in a predefined periodic function, whereby dynamically varying a pitch angle over time results in dynamically varying a pitch angle according to a predefined periodic function.

9. The method of controlling a wind turbine according to claim 8, wherein the periodic tilt function and / or the periodic yaw function is a sine function having a predetermined frequency.

10. 9. The method of controlling a wind turbine according to claim 3, 5 or 8, wherein the predefined periodic function comprises a first sinusoidal function having a first frequency, the first frequency being different from a rotational frequency of the rotor or a multiple thereof.

11. 11. The method of controlling a wind turbine of claim 10, wherein the predefined periodic function comprises a superposition of the first sinusoidal function and a second sinusoidal function having a second frequency, the first and second frequencies being different.

12. 12. The method of controlling a wind turbine according to claim 10 or 11, wherein the first frequency value or the second frequency value is substantially equal to the rotational frequency of the rotor increased or decreased by a predetermined frequency, the predetermined frequency being a non-zero frequency less than the rotational frequency.

13. 13. The method of controlling a wind turbine according to claim 9 or 12, wherein the predetermined frequency is determined in dependence on at least a diameter of the rotor, a rotational speed of the rotor, and / or an inflow wind speed determined upstream of the wind turbine.

14. A method for controlling a wind turbine according to claim 9, 12 or 13, wherein the predetermined frequency is determined in dependence on at least the Strouhal number, the Strouhal number being between 0.05 and 1.

0.

15. The method of controlling a wind turbine according to any one of claims 1 to 14, wherein the difference between the first and second pitch angles is less than or equal to 30°.

16. 1. A wind turbine controller arranged to control a wind turbine including a rotor including at least a first blade, the controller comprising: the controller is arranged to dynamically vary a pitch angle of the first blade over time between a first pitch angle and a second pitch angle while the first blade is rotating, thereby varying the induction coefficient of the first blade over time; the first pitch angle and the second pitch angle are different; a dynamic change in pitch angle over time such that a rotational position of each of the first blades in the rotor plane at a first pitch angle of the first blades is displaced with each revolution, and a rotational position of each of the first blades at a second pitch angle of the first blades is displaced with each revolution, causing a change in the induction coefficient of the blades to occur at different angular positions in the rotor plane over time, whereby a position and / or direction of a wake formed downstream of the wind turbine dynamically changes relative to the rotor of the wind turbine. Wind turbine controller.

17. 20. A wind turbine comprising a rotor including at least a first blade, the wind turbine further comprising the wind turbine controller of claim 16.

18. 17. An array of at least two wind turbines, where for a given wind direction, a second wind turbine is positioned at least partially downstream in a wake of a first wind turbine, the first and second wind turbines including rotors including at least a first blade, and at least the first wind turbine including the wind turbine controller of claim 16.

19. A method of controlling a wind turbine as described in claim 1, wherein dynamic changes in the pitch angle of the first blade over time cause a periodic change in thrust direction for the rotor.

20. The method of controlling a wind turbine as described in claim 2, further comprising imparting both yaw and tilt motion to the rotor, the yaw and tilt motions having a common predetermined frequency and having a predetermined phase offset relative to each other.

21. The wind turbine controller of claim 16, configured such that dynamic changes in the pitch angle of the first blade over time cause a periodic change in thrust direction for the rotor.

22. The wind turbine controller of claim 16, further configured to impart yaw and / or tilt motion to the rotor to obtain forced wake mixing by superimposing a periodic change in pitch angle on the pitch angle of the first blade so as to dynamically change the pitch angle of the first blade.

23. The wind turbine controller of claim 22, further configured to impart both yaw and tilt motion to the rotor, the yaw and tilt motions having a common predetermined frequency and having a predetermined phase offset relative to each other.

24. A method of controlling a wind turbine as described in claim 1, wherein the resulting wake is a helical wake about the axis of the rotor.

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