Determining a reduced thrust mode of operation for a wind turbine

Abstract

The invention relates to determining a reduced thrust coefficient mode of operation for a wind turbine The invention includes retrieving a baseline mode of operation of the wind turbine, defined as a baseline optimal pitch curve having a baseline optimal TSR value. The invention includes retrieving a thrust coefficient table and a power coefficient table. The invention includes defining a maximum thrust coefficient value that is less than a thrust coefficient value of the baseline optimal TSR value in the thrust coefficient table. The invention includes determining, based on the coefficient tables, a maximum thrust coefficient pitch curve that maximises power coefficient value against a constraint that the maximum thrust coefficient value is not exceeded. The invention includes determining the reduced thrust coefficient mode of operation, defined as a reduced thrust coefficient optimal pitch curve determined by modifying the baseline optimal pitch curve using the maximum thrust coefficient pitch curve.

Description

[0001] DETERMINING A REDUCED THRUST MODE OF OPERATION FOR A WIND TURBINE

[0002] TECHNICAL FIELD

[0003] This disclosure relates to the operation of wind turbines, and in particular to a method of determining a reduced thrust coefficient mode of operation for a wind turbine.

[0004] BACKGROUND

[0005] Wind turbines are used to capture energy from wind, and to generate electrical power from the captured energy (for example, to supply to an electrical grid). Wind farms can include several wind turbines in the same location, and may also be referred to as wind parks or wind power plants. Wake influences between wind turbines in a wind farm may have an effect on performance and reliability of the farm. Specifically, the wake of a wind turbine reduces the available energy for downwind turbines to capture, and therefore lowers the annual energy production, AEP, of the wind farm.

[0006] Currently, complex wake simulations may be used to predict wake effects and optimize turbine placement and layout to mitigate its effects. However, these are computationally resource heavy, and require a lot of processing power and memory. Further, these cannot be adapted once the wind turbines are installed in a particular layout. The fixed positions of the turbines will only be optimal under certain environmental conditions, for example in certain wind speeds and directions.

[0007] Pitch control of the rotor blades of a wind turbine is also known, in order to minimize wake losses. However, this approach is sub-optimal because, although wake losses are decreased, the turbine will operate with reduced power. Each wind turbine will therefore be operating at a reduced efficiency, and the AEP of the wind farm will be reduced.

[0008] It is known to operate a wind turbine in accordance with an optimal pitch curve, describing collective pitch angle of the rotor blades as a function of tip-speed ratio of the wind turbine, that maximises power output of the wind turbine. It is also known to operate a wind turbine with reduced thrust, relative to the thrust when operated on the optimal pitch curve, to let more wind pass through the turbine. This may result in reduced wake flow downwind of the turbine in a manner that increases overall power production of a wind farm in which the wind turbine is located. However, it can be relatively time consuming and difficult to create operating modes with reduced thrust for a wind turbine as known methods require several different operating parameters to be tuned manually. The present invention seeks to address these and other disadvantages encountered in the prior art.

[0009] SUMMARY OF THE INVENTION

[0010] According to an aspect of the invention, there is provided a computer implemented method of determining a reduced thrust coefficient mode of operation for a wind turbine. The method comprises retrieving a baseline mode of operation of the wind turbine, defined as a baseline optimal pitch curve that describes collective pitch angle of rotor blades of the wind turbine as a function of tip speed ratio, TSR, of the wind turbine. The baseline optimal pitch curve has a baseline optimal TSR value that maximises power output along the baseline optimal pitch curve. The method comprises retrieving a thrust coefficient table including a thrust coefficient value for each respective pair of a plurality of pairs of collective pitch angle and TSR values. The method comprises retrieving a power coefficient table including a power coefficient value for each respective pair of a plurality of pairs of collective pitch angle and TSR values. The method comprises defining a maximum thrust coefficient value that is not to be exceeded when the wind turbine operates in the reduced thrust coefficient mode. The maximum thrust coefficient value is less than a thrust coefficient value of the baseline optimal TSR value in the thrust coefficient table. The method comprises determining, based on the thrust coefficient table and the power coefficient table, a maximum thrust coefficient pitch curve that describes collective pitch angle as a function of TSR to maximise the power coefficient value when the thrust coefficient value is the defined maximum thrust coefficient value. The method comprises determining the reduced thrust coefficient mode of operation of the wind turbine, defined as a reduced thrust coefficient optimal pitch curve that describes collective pitch angle as a function of TSR. The reduced thrust coefficient optimal pitch curve is determined by modifying the baseline optimal pitch curve using the maximum thrust coefficient pitch curve to maximise power output of the wind turbine against a constraint that the maximum thrust coefficient value is not exceeded.

[0011] The method may comprise determining, based on the power coefficient table, a power coefficient value for each of a plurality of pairs of collective pitch angle and TSR values on the maximum thrust coefficient pitch curve. The method may comprise, based on the determined power coefficient values, determining a reduced thrust coefficient optimal TSR value that maximises power output along the reduced thrust coefficient optimal pitch curve.

[0012] The method may comprise determining, from the thrust coefficient table, a first intermediate point on the maximum thrust coefficient pitch curve having the maximum thrust coefficient value and comprising the baseline optimal TSR value and a corresponding pitch angle. The method may comprise determining, from the thrust coefficient table, a second intermediate point on the maximum thrust coefficient pitch curve having the maximum thrust coefficient value and comprising a TSR value less than the baseline optimal TSR value and a corresponding pitch angle value.

[0013] The method may comprise iteratively determining, from the power coefficient table and for decreasing TSR values, the power coefficient value for each of the plurality of pairs of collective pitch angle and TSR values. The plurality of pairs may comprise the first intermediate point and I or the second intermediate point. The method may comprise, based on the iterative determination of the power coefficient values, determining that a local maximum power coefficient value is reached at the reduced thrust coefficient optimal TSR value.

[0014] The method may comprise determining, from the thrust coefficient table, a starting point on the maximum thrust coefficient pitch curve having the maximum thrust coefficient value and comprising a TSR value and a corresponding pitch angle value that are on the baseline optimal pitch curve.

[0015] The method may comprise retrieving a minimum wind speed value. The method may comprise determining, from the thrust coefficient table, an end point on the maximum thrust coefficient pitch curve having the maximum thrust coefficient value and comprising a TSR value corresponding to the minimum wind speed value and a corresponding pitch angle value.

[0016] Modifying the baseline optimal pitch curve may comprise, for a first range of TSR values, replacing the baseline optimal pitch curve with the maximum thrust coefficient pitch curve. The first range of TSR values may be defined by the starting point on the maximum thrust coefficient pitch curve and the end point on the maximum thrust coefficient pitch curve.

[0017] Modifying the baseline optimal pitch curve may comprise, for a second range of TSR values, replacing the baseline optimal pitch curve with a constant pitch angle. The second range of TSR values may be defined by the end point on the maximum thrust coefficient pitch curve and a point on the baseline optimal pitch curve with the same pitch angle as the end point pitch angle.

[0018] The reduced thrust coefficient mode may be one of a plurality of reduced thrust coefficient modes, each having a respective maximum thrust coefficient that is less than the thrust coefficient value of the baseline optimal TSR value in the thrust coefficient table.

[0019] The maximum thrust coefficient value that is not to be exceeded may be received from a user input device. The method may comprise controlling operation of the wind turbine according to the reduced thrust coefficient mode of operation.

[0020] According to another aspect of the invention there is provided a controller for determining a reduced thrust coefficient mode of operation for a wind turbine. The controller comprises a processor being configured to perform steps according to the method defined above.

[0021] According to another aspect of the invention there is provided a wind turbine comprising a controller as defined above.

[0022] According to another aspect of the invention there is provided a non-transitory, computer- readable storage medium storing instructions thereon that, when executed by one or more computer processors, cause the one or more computer processors to perform the method defined above.

[0023] BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Specific embodiments are now described, by way of example only, with reference to the drawings, in which:

[0025] Figure 1 depicts a wind turbine in accordance with an aspect of the invention;

[0026] Figure 2 depicts how the thrust coefficient varies with wind speed while the wind turbine of Figure 1 operates in a baseline mode of operation;

[0027] Figure 3 depicts a method of operating the wind turbine of Figure 1 in accordance with an aspect of the invention;

[0028] Figure 4 depicts how the thrust coefficient varies with wind speed while the wind turbine of Figure 1 operates in a reduced thrust coefficient mode of operation;

[0029] Figure 5 depicts an exemplary baseline optimal pitch curve and maximum thrust coefficient pitch curve which may be used in the method of Figure 3.

[0030] DETAILED DESCRIPTION

[0031] In overview, the invention provides a computer-implemented method of determining a reduced thrust coefficient mode of operation for a wind turbine. A mode of operation of a wind turbine may be defined by an optimal pitch curve describing the collective pitch angle of rotor blades of the wind turbine as a function of tip speed ratio (TSR) of the wind turbine. The optimal pitch curve is a curve that tracks the maximum power points of a wind turbine. A wind turbine can be operated according to its optimal pitch curve by controlling its rotor speed to adjust tip speed ratio, and by controlling the angle of at least one rotor blade of the turbine.

[0032] A baseline mode of operation may involve operating a wind turbine according to a baseline optimal pitch curve that is defined to maximise power output for different TSR values (without any constraint on wind turbine thrust). The baseline optimal pitch curve includes a baseline optimal point having a baseline optimal TSR value at which power output of the wind turbine is maximised along the baseline optimal pitch curve.

[0033] The thrust coefficient (CT) of a wind turbine is a non-dimensional parameter that is a measure of the axial force exerted by the wind on the rotor. The thrust force is indicative of the relative size and intensity of the turbine wake, and therefore the energy that remains in the wind after it has passed the turbine. Operating a given wind turbine in a reduced thrust coefficient mode therefore reduces the maximum permitted force exerted on the wind and the resultant wake, relative to a baseline mode in which power output is maximised without any constraint on the thrust. This can increase the overall AEP for a wind farm, as each turbine may be more easily able to capture the available energy without overly reducing the energy for subsequent downwind turbines.

[0034] A reduced thrust coefficient mode of operation may be provided by adapting or modifying the baseline optimal pitch curve of the baseline mode of operation to determine a reduced thrust optimal pitch curve. This modifying uses a so-called maximum thrust coefficient pitch curve which describes collective pitch angle as a function of TSR in a manner in which the power coefficient value is maximised while constraining the maximum thrust coefficient value to a value that is less than the thrust coefficient for the baseline optimal TSR value.

[0035] The maximum thrust coefficient pitch curve may replace the baseline optimal pitch curve for a particular range of TSR values. The maximum thrust coefficient pitch curve is determined using a thrust coefficient table having a thrust coefficient value for each respective pair of a plurality of pairs of collective pitch angle and TSR values and a power coefficient table having a power coefficient value for each respective pair of a plurality of pairs of collective pitch angle and TSR values.

[0036] In some implementations, points on the maximum thrust coefficient pitch curve may be determined for a plurality of TSR values, starting from the baseline optimal TSR value and decreasing the TSR value iteratively. A reduced thrust coefficient optimal point having a reduced thrust coefficient optimal TSR value may be calculated by determining a power coefficient value for a plurality of points on the maximum thrust coefficient pitch curve, using the power coefficient table. The TSR value at which there is a local maximum in the power coefficient value can therefore be found. This may be an iterative process. This will be discussed in greater detail below.

[0037] In some implementations, the computer-implemented methods of the present disclosure may be performed on a processor remote from the wind turbine. The determined reduced thrust coefficient mode of operation may subsequently be provided to a wind turbine controller for controlling operation of the wind turbine according to said determined reduced thrust coefficient mode of operation. In other implementations, the computer-implemented methods of the present disclosure may be performed on a processor at the wind turbine itself. In other words, the controller of the wind turbine may perform the steps for determining a reduced thrust coefficient mode of operation and may be further configured to control operation of the wind turbine based on the reduced thrust coefficient mode of operation.

[0038] The maximum thrust coefficient value that is not to be exceeded may be set by user input at a computing device. For example, the user may select or input a thrust coefficient value, or they may select or input an integer multiplier for a pre-set interval below the thrust coefficient value of the baseline optimal TSR value. There may be provided a plurality of reduced thrust coefficient modes, each having a respective maximum thrust coefficient that is less than the thrust coefficient value of the baseline optimal TSR value in the thrust coefficient table and optionally each corresponding to an integer multiplier of the pre-set interval by which the thrust value is reduced.

[0039] The methods described herein may be repeated to determine a plurality of reduced thrust coefficient modes. There may therefore be provided, for example to a wind turbine controller, a plurality of respective optimal pitch curves for a first thrust coefficient mode, a second thrust coefficient mode, and so on. These may be provided in addition to a baseline optimal pitch curve. Optionally, a user may select one of the plurality of reduced thrust coefficient modes, and the corresponding optimal pitch curve may be provided to the controller to control operation of the wind turbine.

[0040] Figure 1 illustrates, in a schematic view, an example of a wind turbine 10. The wind turbine 10 includes a tower 102, a nacelle 103 disposed at the apex of, or atop, the tower 102, and a rotor 104 operatively coupled to a generator housed inside the nacelle 103. In addition to the generator, the nacelle 103 houses other components required for converting wind energy into electrical energy and various components needed to operate, control, and optimise the performance of the wind turbine 10. The rotor 104 of the wind turbine 10 includes a central hub 105 and three rotor blades 106 that project outwardly from the central hub 105. The rotor blades 106 are pitch-adjustable. In other words, the pitch angle of the rotor blades can change. The rotor blades 106 can be adjusted in accordance with a collective pitch setting, where each of the blades are set to the same pitch value. The rotor blades 106 may additionally be adjustable in accordance with individual pitch settings, where each blade 106 may be provided with an individual pitch setpoint. The wind turbine 10 may be part of a wind farm comprising a plurality of wind turbines.

[0041] The methods described herein may be implemented by a controller or other processing module associated with the wind turbine 10. In some examples, the controller or processing module may be located in the wind turbine 10, e.g. inside the nacelle 103, in the tower 102 or distributed at a number of locations inside the turbine 10 and communicatively connected to one another. Alternatively, the controller (or part thereof) may be located externally to the wind turbine 10. The controller may be in the form of any suitable computing device, for instance one or more functional units or modules implemented on one or more computer processors. Such functional units may be provided by suitable software running on any suitable computing substrate using conventional or custom processors and memory. The one or more functional units may use a common computing substrate (for example, they may run on the same server) or separate substrates, or one or both may themselves be distributed between multiple computing devices. A computer memory may store instructions for performing the methods performed by the controller, and the processor(s) may execute the stored instructions to perform the method.

[0042] The described method makes use of the relationships between tip speed ratio, pitch angle, thrust coefficient, and power coefficient in order to determine a reduced thrust coefficient mode of operation. Although the methods herein are described with reference to TSR values and collective pitch angle values, in another implementation, other suitable parameters such as rotor speed, generator speed, torque or torque coefficients may be used. TSR may optionally be controlled by adjusting one or more of these alternative parameters.

[0043] The rotor speed of the rotor 104 can be adjusted to change a tip speed ratio (TSR) for example to maintain an optimal TSR to achieve optimal power at various wind speeds. Tip speed ratio is the ratio between the tangential speed of the tip of a blade of the wind turbine, and the actual speed of the wind.

[0044] The thrust coefficient CTand the power coefficient CPare dimensionless performance parameters of a wind turbine. Both thrust coefficient and power coefficient vary with tip speed ratio and pitch angle of the rotor blades. A given TSR value and pitch angle value pair will have an associated thrust coefficient value and power coefficient value. Figure 2 depicts a plot of power output and thrust coefficient against wind speed when the wind turbine 10 is operated according to a baseline optimal pitch curve. The plot for thrust coefficient is shown with a dashed line 210, and the plot for power output is shown with a solid line 220.

[0045] The cut-in wind speed is the wind speed at which the wind turbine will begin to generate electricity. It will vary for different turbines, depending on a number of factors such as the size and shape, but is typically around 3 metres per second.

[0046] The plots of power output and thrust coefficient in Figure 2 begin at the cut-in wind speed.. At a rated wind speed, the wind turbine is producing the maximum power, i.e. rated power. There is a section of the thrust coefficient plot 210 of Figure 2 where the thrust coefficient is substantially constant. This section is in the partial load region of operation i.e., below the rated wind speed. In the partial load region, the thrust coefficient is higher than in the full load region.

[0047] The thrust coefficient is inversely related to the wake effect that slows the wind velocity as it interacts with a given wind turbine. When the wind turbine is operated under partial load conditions, wake losses have the most impact and overall AEP of the wind farm will be most affected. When the wind turbine is operating in a full load region of operation and producing maximum power, the wake losses will therefore be less significant. Operation at a reduced thrust coefficient in partial load conditions can therefore lead to significantly reduced wake losses.

[0048] It is advantageous to operate in a reduced thrust coefficient mode - with a limit on thrust coefficient - rather than limiting rotor thrust itself, for example by setting a limit on rotor speed and / or pitch angle. Limiting rotor thrust may reduce mechanical loads on the individual wind turbine that is being operated, but reduced thrust operation does not reduce wake losses as effectively as reduced thrust coefficient modes of operation. Thrust limitation typically acts around rated wind speed, whereas examples of the invention beneficially provides reduced thrust coefficient modes of operation that can be implemented for a range of wind speeds, e.g. wind speeds for which the wind turbine operates under partial load where small changes in collective pitch angle can cause changes in thrust.

[0049] Figure 3 depicts the steps of a method 30 for determining a reduced thrust coefficient mode of operation. Method 30 may be performed by a computer processor, and in some implementations, performed by a processor of a controller of the wind turbine 10, in accordance with examples of the invention. At step 310, a baseline mode of operation of the wind turbine 10 is retrieved. As described above, the baseline mode of operation is defined as a baseline optimal pitch curve having a baseline optimal TSR value.

[0050] The baseline optimal pitch curve describes collective pitch angle of rotor blades of the wind turbine as a function of tip speed ratio of the wind turbine. It may be provided as a look-up table of pairs of TSR and pitch angle values. The baseline optimal TSR value is the TSR value that corresponds to a point on the baseline optimal pitch curve (with a corresponding pitch angle value) that maximises power output along the baseline optimal pitch curve.

[0051] At step 320, a thrust coefficient table is retrieved.

[0052] The thrust coefficient table includes a thrust coefficient value for each respective pair of a plurality of pairs of collective pitch angle and TSR values. In some implementations, the thrust coefficient table may comprise a thrust coefficient surface defined by thrust coefficient isolines for a plurality of thrust coefficient values. In other words, a plurality of pairs of collective pitch angle and TSR values will have the same thrust coefficient value, forming an isoline which may be defined by a function.

[0053] In some implementations, the values in the look-up table may be calculated based on one fixed wind speed and / or one fixed air density value. In practice, torsion in the blades 106 means that despite achieving a constant TSR and pitch, the thrust coefficient will change as wind speed changes. For example, an increase in wind speed causes greater blade torsion and therefore changes the apparent pitch angle along the blades. Increasing torsional stiffness of the blades creates a flatter thrust coefficient curve i.e., one where the thrust coefficient does not change as much with wind speed.

[0054] Optionally, the thrust coefficient value for the baseline optimal point having a baseline optimal TSR value can be found from the thrust coefficient look-up table. This baseline optimal TSR value is a fixed value for a given turbine, and the turbine may be regulated to this point during variable rotor speed operation such that, as the wind speed varies, the rotor speed varies to maintain maximum power output of the turbine. The optimal TSR value will vary for different wind turbine designs, specifically with regard to the blade design. However, as discussed elsewhere, this baseline optimal TSR value optimises power production for the wind turbine 10 but may not be optimal for reducing wake losses and increasing the overall AEP of the wind farm of which the wind turbine 10 is part, particularly when the wind turbine 10 is operating under partial load.

[0055] At step 330, a power coefficient table is retrieved. The power coefficient table includes a power coefficient value for each respective pair of a plurality of pairs of collective pitch angle and TSR values. In some implementations, the power coefficient table may comprise a power coefficient surface defined by power coefficient isolines for a plurality of power coefficient values. Each isoline may be defined by a function and / or a plurality of pairs of collective pitch angle and TSR values that have the same power coefficient value.

[0056] The power coefficient surface and / or the thrust coefficient surface may be described as performance maps of parameters such as pitch angle, rotor speed, or tip speed ratio. The power coefficient surface and / or the thrust coefficient surface may also map other parameters such as rotor speed, generator speed, torque or torque coefficients.

[0057] The baseline mode of operation, and / or power coefficient table, and / or the thrust coefficient table may be retrieved from the memory of a computing device. A value may be requested from the look up table stored in memory (either local or remote to the processor carrying out method 30), and subsequently received. For example, a power coefficient value can be retrieved from the power coefficient table following a request for a given pitch angle and TSR value pair. Similarly, a pitch angle value could be retrieved for a given TSR value and a desired thrust coefficient.

[0058] At step 340, a maximum thrust coefficient value that is not to be exceeded when the wind turbine operates in the reduced thrust coefficient mode is defined. The maximum thrust coefficient value is less than a thrust coefficient value of the baseline optimal TSR value in the thrust coefficient table. The maximum thrust coefficient value may be received via a user input at a computing device.

[0059] At step 350, a maximum thrust coefficient pitch curve is determined based on the thrust coefficient table and the power coefficient table.

[0060] The maximum thrust coefficient pitch curve describes collective pitch angle as a function of TSR to maximise the power coefficient value when the thrust coefficient value is the defined maximum thrust coefficient value. An example method for determining the maximum thrust coefficient pitch curve is described in detail below.

[0061] Although it may be desirable to operate in a reduced thrust coefficient mode, particularly for partial load operation, reducing the thrust coefficient will also lower the power coefficient accordingly. It is particularly advantageous to provide a reduced thrust coefficient mode of operation that maximises the power coefficient while also being constrained by the maximum thrust coefficient. Operating the turbine in such a reduced thrust coefficient mode of operation therefore balances the potential wake losses and the power production of a given wind turbine. At step 360, the reduced thrust coefficient mode of operation of the wind turbine is determined. The reduced thrust coefficient mode of operation is defined as a reduced thrust coefficient optimal pitch curve, which is determined by modifying the baseline optimal pitch curve using the maximum thrust coefficient pitch curve.

[0062] Modifying the baseline optimal pitch curve may involve replacing at least part of the curve with the maximum thrust coefficient pitch curve. At least one region of the baseline optimal pitch curve may be unmodified when determining the reduced thrust coefficient optimal pitch curve. This implementation of method 30 will be discussed in more detail below.

[0063] It is more computationally efficient to modify the known baseline optimal pitch curve than to determine each point on the reduced thrust coefficient curve by optimizing for power across the entire range of TSR values. Indeed, the thrust coefficient value at some points along the baseline optimal pitch curve will be less than the defined maximum thrust coefficient value of the reduced thrust coefficient mode being considered. As some points remain unmodified, fewer computational resources are required.

[0064] Optionally, the method may further comprise determining a reduced thrust coefficient optimal TSR value that maximises power output along the reduced thrust coefficient optimal pitch curve. The wind turbine 10 may be provided with the reduced thrust coefficient optimal pitch curve and / or the reduced thrust coefficient optimal point in order to be able to operate the wind turbine 10 according to the respective reduced thrust coefficient mode.

[0065] Figure 4 depicts several plots of thrust coefficient against wind speed when the wind turbine 10 is operated according to a variety of different optimal pitch curves corresponding to different modes of operation. These may be referred to as thrust coefficient curves.

[0066] Plot 210 shows the thrust coefficient when the wind turbine 10 is operated according to a baseline optimal pitch curve, as in Figure 2. Plots 420, 430, 440, 450 correspond to operation according to a first, second, third and fourth reduced thrust coefficient optimal pitch curve, respectively. Each of the first to fourth reduced thrust coefficient optimal pitch curves 420-450 is associated with a corresponding maximum thrust coefficient value. The maximum thrust coefficient for the fourth reduced thrust coefficient optimal pitch curve 450 is less than the maximum thrust coefficient for the third reduced thrust coefficient optimal pitch curve 440, which is in turn less than the maximum thrust coefficient for the second reduced thrust coefficient optimal pitch curve 430, and which is in turn less than the maximum thrust coefficient for the first reduced thrust coefficient optimal pitch curve 420.

[0067] When the wind turbine 10 is operated according to reduced thrust coefficient modes, the maximum thrust coefficient of each plot 420, 430, 440, 450 is reduced for a section of the curve corresponding to partial load operation. The thrust coefficient for each of the reduced curves - as indicated by plots 420, 430, 440, 450 - is the same as the thrust coefficient for the baseline curve - as indicated by plot 210 - at the cut-in wind speed 460. A minimum wind speed 470, greater than the cut-in wind speed 460, is the minimum value of wind speed for which the described method is used to determine a reduced thrust coefficient mode of operation of the wind turbine. The minimum wind speed is any suitable wind speed value. Above the rated wind speed, in the full load region, the thrust coefficient curves 210, 420, 430, 440, 450 converge. In this full load region, the thrust coefficient is below the maximum thrust coefficient for any of the first to fourth reduced thrust coefficient modes of operation.

[0068] Figure 5 depicts two tables, one being a table illustrating a power coefficient, Cp, surface 580 and another being a table illustrating a thrust coefficient, CT, surface 590. Both the Cpsurface 580 and the CT surface 590 are depicted by a plurality of isolines. Figure 5 depicts an example of a baseline optimal pitch curve made up of a first section 502, a second section 504 and a third section 506. It also depicts an example of a maximum thrust coefficient pitch curve 508 having a specified (maximum) thrust coefficient value. The baseline optimal point 510 on the baseline optimal pitch curve 502, 504, 506 has the baseline optimal TSR value. The reduced thrust coefficient optimal point 520 on the maximum thrust coefficient pitch curve 508 has the reduced thrust coefficient optimal TSR value.

[0069] Some exemplary points are shown on the second section 504 of the baseline optimal pitch curve. Other exemplary points are shown on the first section 502 and third section 506 of the baseline optimal pitch curve, and these points are common to the baseline optimal pitch curve 502, 504, 506 and a reduced thrust coefficient pitch curve, as will be discussed in more detail below.

[0070] The isolines of the power coefficient, Cp, surface 580 and the thrust coefficient, CT, surface 590 visually represent look-up tables of Cpand CT values. Each isoline corresponds to a specific value of Cpor CT. For a given coefficient value, there are many collective pitch angles and TSR values that satisfy that constraint. It can be seen from Figure 5 that the points on the maximum thrust coefficient pitch curve 508 have the maximum thrust coefficient value. If further reduced thrust coefficient modes of operations were to be determined, they would correspond to different isolines having thrust coefficients that are reduced relative to the isoline that the baseline optimal point 510 sits on. In other words, they would have a maximum thrust coefficient less than the thrust coefficient of the baseline optimal point 510. Modifying the baseline optimal pitch curve 502, 504, 506 may comprise receiving or determining the baseline optimal point 510. In other words, it may involve determining the TSR value on the baseline optimal pitch curve at which power is maximised. Optionally, this may comprise requesting and subsequently retrieving from the power coefficient table, the TSR and pitch angle value pair that has a maximum power coefficient and lies on the optimal pitch curve. The baseline optimal point is typically fixed for a given wind turbine and may typically have a TSR value between 7 and 10, for instance.

[0071] Optionally, modifying the baseline optimal pitch curve 502, 504, 506 may further comprise determining a first intermediate point 530 having the same TSR value as the baseline optimal point 510. From the thrust coefficient table or the thrust coefficient surface, a collective pitch angle value is determined that satisfies the constraint of having a thrust coefficient less than or equal to the maximum thrust coefficient value and the baseline optimal TSR value. This first intermediate point 530 may be the first point calculated to form the maximum thrust coefficient pitch curve 508.

[0072] Further points on the maximum thrust coefficient pitch curve 508 may be calculated. For example, a second intermediate point 540 may be determined for a TSR value less than the baseline optimal TSR value. As above, a corresponding pitch angle may be retrieved from the thrust coefficient table or the thrust coefficient surface, applying the maximum thrust coefficient value constraint and the desired TSR value. The second intermediate point may have a TSR value between the baseline optimal TSR value and the TSR value of the reduced thrust coefficient optimal point 520.

[0073] This may be an iterative process, determining a plurality of points at TSR values with constant intervals, starting from the baseline optimal TSR value and subsequently decreasing the TSR value. The process may further involve retrieving, from the power coefficient table or power coefficient surface, a power coefficient for each iteratively determined point as it is identified. In some implementations, the iterative process may continue until the power coefficient value begins to decrease. In this way, local power maxima can be identified.

[0074] Optionally, it may be determined that a local maximum power coefficient value has been reached at the reduced thrust coefficient optimal TSR value, when the power coefficient values for points on either side of the reduced thrust optimal point 520 are lower than at the maximum.

[0075] In some implementations, there may be more than one maximum power coefficient in the maximum thrust coefficient pitch curve 508. Further maxima may be identified by continuing to iteratively determine more points on the maximum thrust coefficient pitch curve 508. Preferably, it is the first maximum point (having a TSR value closer to the baseline optimal TSR value) that is selected as the reduced thrust coefficient optimal point 520. This is advantageous because the first maximum requires a smaller change in tip speed ratio, and therefore the change to the rotor speed is minimised. It is therefore a more efficient method of adjusting the rotor conditions. It also reduces the chances of the maximum being a mathematical maximum but giving rise to various other inefficiencies.

[0076] Alternatively or additionally, modifying the baseline optimal pitch curve 502, 504, 506 may comprise determining a starting point 550 and I or an end point 560 of the maximum thrust coefficient pitch curve 508. In some implementations, rather than begin at the baseline optimal TSR value, a power coefficient value may be determined from the power coefficient table for TSR and pitch angle value pairs at regular TSR intervals between the starting point 550 and the end point 560. Optionally, both methods may be carried out, and the determined reduced thrust coefficient optimal TSR values may be compared to each other. For example, a mean optimal TSR value for the reduced thrust coefficient mode of operation may be found. A starting point 550 of the maximum thrust coefficient pitch curve 508 has a TSR value and a corresponding pitch angle value that lies on the baseline optimal pitch curve 502, 504, 506. Starting point 550 also has the maximum thrust coefficient value. In other words, this is the point where the baseline optimal pitch curve 502, 504, 508 intersects the thrust coefficient isoline for the maximum thrust coefficient value.

[0077] In some implementations, the starting point 550 may be determined by iteratively determining the thrust coefficient for points on the first section of the baseline optimal pitch curve 502 from the thrust coefficient table until a point 550 is determined that has the maximum thrust coefficient value. In other implementations, it may be determined by iteratively determining the points on the maximum thrust coefficient pitch curve 508, for example, beginning at the baseline optimal TSR value at point 530 and decreasing the TSR value in increments until a point 550 is determined that is on the baseline optimal pitch curve.

[0078] An end point 560 of the maximum thrust coefficient pitch curve 508 has a TSR value corresponding to the minimum wind speed value and a corresponding pitch angle value. The tip speed ratio may be calculated using the selected minimum wind speed value for the turbine, a minimum angular rotor speed, and the radius of the rotor. This end point parameter may be stored in memory and retrieved.

[0079] Preferably, the baseline optimal pitch curve is replaced by at least a portion of the maximum thrust coefficient pitch curve 508. A first region of the maximum thrust coefficient pitch curve 508 may be defined by the TSR values between the starting point 550 and the end point 560. The first region of the maximum thrust coefficient pitch curve 508 may replace the corresponding TSR values of the baseline optimal pitch curve.

[0080] Additionally, or alternatively, a second region of the maximum thrust coefficient pitch curve 508 may be defined by the TSR values between the end point 560 (i.e. , the minimum wind speed TSR) and a point 570 on the baseline optimal pitch curve 502, 504, 506 with the same pitch angle as the end point pitch angle. In this second region, the pitch angle is constant and equal to the end point pitch angle. The second region of the maximum thrust coefficient pitch curve 508 may replace the corresponding TSR values of the baseline optimal pitch curve.

[0081] At the starting point 550, the baseline optimal pitch curve 502, 504, 506 and the maximum thrust coefficient pitch curve 508 intersect, and therefore at TSR values below the starting point 550, the baseline optimal pitch curve is unmodified. In other words, the first section 502 of the baseline optimal pitch curve is unmodified, and the points on that section are common to both the baseline optimal pitch curve 502, 504, 506 and the reduced thrust coefficient optimal pitch curve.

[0082] Likewise, above point 570 where the maximum thrust coefficient pitch curve 508 intersects with the baseline optimal pitch curve 502, 504, 506, the baseline optimal pitch curve is unmodified. In other words, the third section 506 of the baseline optimal pitch curve is also unmodified. The points on the third section 506 therefore may be common to both the baseline optimal pitch curve 502, 504, 506 and the reduced thrust coefficient optimal pitch curve.

[0083] Preferably, the reduced thrust coefficient optimal pitch curve may comprise the first section 502, the maximum thrust coefficient pitch curve 508 between starting point 550 and point 570, and the third section 506 of the baseline optimal pitch curve.

[0084] Below the minimum speed associated with end point 560, any downstream wind turbine in a wind farm will not be affected much as a result of the low wind speed, and therefore the benefit of operating in a CT mode is limited.

[0085] It will be understood that the above description of specific embodiments is by way of example only and is not intended to limit the scope of the present disclosure. Many modifications of the described embodiments, some of which are now described, are envisaged and intended to be within the scope of the present disclosure.

Claims

CLAIMS1. A computer implemented method of determining a reduced thrust coefficient mode of operation for a wind turbine, the method comprising: retrieving a baseline mode of operation of the wind turbine, defined as a baseline optimal pitch curve that describes collective pitch angle of rotor blades of the wind turbine as a function of tip speed ratio, TSR, of the wind turbine, the baseline optimal pitch curve having a baseline optimal TSR value that maximises power output along the baseline optimal pitch curve; retrieving a thrust coefficient table including a thrust coefficient value for each respective pair of a plurality of pairs of collective pitch angle and TSR values; retrieving a power coefficient table including a power coefficient value for each respective pair of a plurality of pairs of collective pitch angle and TSR values; defining a maximum thrust coefficient value that is not to be exceeded when the wind turbine operates in the reduced thrust coefficient mode, wherein the maximum thrust coefficient value is less than a thrust coefficient value of the baseline optimal TSR value in the thrust coefficient table; determining, based on the thrust coefficient table and the power coefficient table, a maximum thrust coefficient pitch curve that describes collective pitch angle as a function of TSR to maximise the power coefficient value when the thrust coefficient value is the defined maximum thrust coefficient value; and determining the reduced thrust coefficient mode of operation of the wind turbine, defined as a reduced thrust coefficient optimal pitch curve that describes collective pitch angle as a function of TSR, the reduced thrust coefficient optimal pitch curve being determined by modifying the baseline optimal pitch curve using the maximum thrust coefficient pitch curve to maximise power output of the wind turbine against a constraint that the maximum thrust coefficient value is not exceeded.

2. A computer implemented method according to claim 1 , further comprising: determining, based on the power coefficient table, a power coefficient value for each of a plurality of pairs of collective pitch angle and TSR values on the maximum thrust coefficient pitch curve; andbased on the determined power coefficient values, determining a reduced thrust coefficient optimal TSR value that maximises power output along the reduced thrust coefficient optimal pitch curve.

3. A computer implemented method according to any preceding claim, further comprising: determining, from the thrust coefficient table, a first intermediate point on the maximum thrust coefficient pitch curve having the maximum thrust coefficient value and comprising the baseline optimal TSR value and a corresponding pitch angle.

4. A computer implemented method according to any preceding claim, further comprising: determining, from the thrust coefficient table, a second intermediate point on the maximum thrust coefficient pitch curve having the maximum thrust coefficient value and comprising a TSR value less than the baseline optimal TSR value and a corresponding pitch angle value.

5. A computer implemented method according to any of claims 2 to 4, further comprising: iteratively determining, from the power coefficient table and for decreasing TSR values, the power coefficient value for each of the plurality of pairs of collective pitch angle and TSR values, wherein the plurality of pairs comprise the first intermediate point and I or the second intermediate point; and based on the iterative determination of the power coefficient values, determining that a local maximum power coefficient value is reached at the reduced thrust coefficient optimal TSR value.

6. A computer implemented method according to any preceding claim, further comprising: determining, from the thrust coefficient table, a starting point on the maximum thrust coefficient pitch curve having the maximum thrust coefficient value and comprising a TSR value and a corresponding pitch angle value that are on the baseline optimal pitch curve.

7. A computer implemented method according to any preceding claim, further comprising: retrieving a minimum wind speed value; determining, from the thrust coefficient table, an end point on the maximum thrust coefficient pitch curve having the maximum thrust coefficient value and comprising a TSR value corresponding to the minimum wind speed value and a corresponding pitch angle value.

8. A computer implemented method according to claim 6 and 7, wherein modifying the baseline optimal pitch curve comprises, for a first range of TSR values, replacing the baseline optimal pitch curve with the maximum thrust coefficient pitch curve, wherein the first range of TSR values is defined by the starting point on the maximum thrust coefficient pitch curve and the end point on the maximum thrust coefficient pitch curve.

9. A computer implemented method according to claim 8, wherein modifying the baseline optimal pitch curve further comprises, for a second range of TSR values, replacing the baseline optimal pitch curve with a constant pitch angle, wherein the second range of TSR values is defined by the end point on the maximum thrust coefficient pitch curve and a point on the baseline optimal pitch curve with the same pitch angle as the end point pitch angle.

10. The computer implemented method of any preceding claim, wherein the reduced thrust coefficient mode is one of a plurality of reduced thrust coefficient modes, each having a respective maximum thrust coefficient that is less than the thrust coefficient value of the baseline optimal TSR value in the thrust coefficient table.

11. The computer implemented method of any preceding claim, wherein the maximum thrust coefficient value that is not to be exceeded is received from a user input device.

12. A computer implemented method according to any preceding claim, further comprising controlling operation of the wind turbine according to the reduced thrust coefficient mode of operation.

13. A controller for determining a reduced thrust coefficient mode of operation for a wind turbine, the controller comprising a processor being configured to perform steps according to any of claims 1 to 12.

14. A wind turbine comprising a controller according to claim 13.

15. A computer-readable medium for performing the method of any of claims 1 to 12.

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