Controlling a wind turbine operating in a noise control mode to mitigate stall

Abstract

The invention relates to controlling a wind turbine that is operating in a noise control mode. A baseline minimum pitch curve is retrieved, that describes collective pitch angle of rotor blades of the wind turbine as a function of tip-speed ratio, TSR, of the wind turbine, to maximise power output. A defined stall angle is retrieved that is an angle of attack of the wind turbine for stalling the wind turbine. A maximum angle of attack that is a defined margin less than the defined stall angle is set. The invention includes determining a modified minimum pitch curve that modifies the baseline minimum pitch (20) curve to maximise power output of the wind turbine against a constraint that the defined maximum angle of attack is not exceeded. The invention includes controlling the wind turbine to operate in the noise control mode according to the modified minimum pitch curve.

Description

[0001] CONTROLLING A WIND TURBINE OPERATING IN A NOISE CONTROL MODE TO

[0002] MITIGATE STALL

[0003] TECHNICAL FIELD

[0004] The invention relates to controlling a wind turbine operating in a noise control mode to mitigate stall. In particular, the wind turbine is controlled in accordance with a minimum blade pitch curve describing a relationship between a collective pitch angle and a tip-speed ratio so as not to exceed a defined maximum angle of attack.

[0005] BACKGROUND

[0006] Wind turbines are known to comprise pitch adjustable rotor blades, a generator, and, optionally, a gearbox. The rotor blades are used to capture energy in the wind as it flows past them, and the generator is used to generate electrical power from the captured energy, e.g. to be supplied to an electrical grid. Wind turbines operate in a variety of conditions, e.g. environmental conditions, such as different wind speeds.

[0007] In certain conditions, for example turbulent wind conditions or low wind speeds, wind turbines are often operated in accordance with a control scheme designed to limit the rotational velocity of the rotor blades. For instance, in relatively low wind speeds a wind turbine may be operated to limit the rotational velocity of the rotor blades in order to reduce emitted noise. Noise may be generated by rotating rotor blades and adjusting wind turbine operation to reduce the emitted noise may include reducing the rotational speed of the rotor and rotor blades.

[0008] Controlling a wind turbine to maximise power output may be performed using minimum pitch curves describing a relationship between collective pitch angle and tip-speed ratio. For certain tip-speed ratio values, operating a wind turbine according to its minimum pitch curve increases the risk of stall. In derated modes of operation, such as in some noise reduction modes, a wind turbine may be operated according to a minimum pitch curve with a tip-speed ratio value that increases the risk of the wind turbine stalling.

[0009] It is against this background to which the present invention is set. SUMMARY OF THE INVENTION

[0010] According to an aspect of the invention there is provided a method of controlling a wind turbine that is operating in a noise control mode. The method comprises retrieving a baseline minimum 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, to maximise power output of the wind turbine. The method comprises retrieving a defined stall angle that is an angle of attack of the wind turbine for stalling the wind turbine. The method comprises defining a maximum angle of attack that is a defined margin less than the defined stall angle. The method comprises determining a modified minimum pitch curve that modifies the baseline minimum pitch curve to maximise power output of the wind turbine against a constraint that the defined maximum angle of attack is not exceeded. The method comprises controlling the wind turbine to operate in the noise control mode according to the modified minimum pitch curve.

[0011] The noise control mode may be a mode in which the wind turbine is controlled to operate at a derated rotor speed, relative to the rotor speed that would be used if the turbine were not in noise control mode. In other words, the noise control mode involves operating the turbine at a rotor speed lower than the reference rotor speed used for normal mode control, the refence rotor speed being the nominal speed under normal conditions. In one embodiment, the noise control mode may also include a power derate in addition to the speed derate. Such power derate may be applied to ensure compliance with a torque limit. The maximum rotor speed in noise control mode may be selected to ensure that emitted noise does not exceed permitted levels. Based on this maximum rotor speed, a resulting power derate may be set to maintain compliance with a torque limit.

[0012] Determining the modified minimum pitch curve may comprise modifying the baseline minimum pitch curve only for TSR values within a defined TSR range of TSR values.

[0013] The defined TSR range may not include a defined optimal TSR value of the wind turbine.

[0014] The defined TSR range may include only TSR values less than the defined optimal TSR value. The method may comprise controlling the wind turbine to operate according to the modified minimum pitch curve in a constant speed partial load region of a defined power curve of the wind turbine.

[0015] Determining the modified minimum pitch curve may comprise executing a plurality of wind turbine simulations in which the wind turbine is operated in the noise control mode.

[0016] Executing each wind turbine simulation may comprise defining a respective simulation minimum pitch curve describing collective pitch angle as a function of TSR of the wind turbine. Executing each wind turbine simulation may comprise controlling the wind turbine to operate according to the respective simulation minimum pitch curve for different wind speeds. Executing each wind turbine simulation may comprise obtaining a pair of collective pitch angle and angle of attack values for each of a plurality of wind speeds as output from the wind turbine simulation. Executing each wind turbine simulation may comprise determining angle of attack as a function of TSR to maximise angle of attack against the constraint that the defined maximum angle of attack is not exceeded. Determining the angle of attack as a function of TSR may be based on the obtained pair of values and the simulation minimum pitch curve of each of the plurality of wind turbine simulations. Executing each wind turbine simulation may comprise determining collective pitch angle as a function of TSR based on the determined angle of attack as a function of TSR to determine the modified minimum pitch curve.

[0017] Defining the respective simulation minimum pitch curve may comprise offsetting collective pitch angle of the baseline minimum pitch curve by a respective positive value.

[0018] Offsetting the collective pitch angle of the baseline minimum pitch curve by a respective positive value may include increasing the collective pitch angle by a constant positive value over the full range of TSR values. In particular, offsetting the collective pitch angle may involve increasing the pitch angle away from a stall point, that is increasing the pitch angle towards a feather position of the blade.

[0019] Determining collective pitch angle may comprise determining a map of the obtained pairs of collective pitch angle and angle of attack values for each of the wind turbine simulations and interpolating the collective pitch angle values in the determined map to maximise angle of attack against the constraint that the defined maximum angle of attack is not exceeded. The modified minimum pitch curve may be determined based on the interpolated collective pitch angle values.

[0020] The defined TSR range may include a defined maximum TSR value and the method may comprise interpolating the modified minimum pitch curve at TSR values less than the defined maximum TSR value with the baseline minimum pitch curve at TSR values greater than the defined maximum TSR value to determine the modified minimum pitch curve.

[0021] The maximum angle of attack may be defined based on a determined wind condition in which the wind turbine is operating.

[0022] The method may comprise receiving sensor data, from one or more sensors of the wind turbine, indicative of loading on one or more components of the wind turbine. The received sensor data may be indicative of a tilt / yaw moment on a rotor of the wind turbine. Determining the wind condition in which the wind turbine is operating may be based on the received sensor data.

[0023] When the wind condition is determined to be a benign wind condition then the method may comprise controlling the wind turbine to operate in accordance with the baseline minimum pitch curve.

[0024] When the wind condition is a non-benign wind condition the maximum angle of attack may be defined to be less than the maximum angle of attack when the wind condition is a benign wind condition. The non-benign wind condition may be for a wind shear condition above a certain level and / or for a wind turbulence condition above a certain turbulence intensity.

[0025] Controlling the wind turbine may comprise adjusting a blade pitch reference or an output power reference of the wind turbine.

[0026] According to another aspect of the invention there is provided a controller for controlling a wind turbine that is operating in a noise control mode. The controller is configured to retrieve a baseline minimum 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, to maximise power output of the wind turbine. The controller is configured to retrieve a defined stall angle that is an angle of attack of the wind turbine for stalling the wind turbine. The controller is configured to define a maximum angle of attack that is a defined margin less than the defined stall angle. The controller is configured to determine a modified minimum pitch curve that modifies the baseline minimum pitch curve to maximise power output of the wind turbine against a constraint that the defined maximum angle of attack is not exceeded. The controller is configured to control the wind turbine to operate in the noise control mode according to the modified minimum pitch curve.

[0027] According to another aspect of the invention there is provided a wind turbine comprising a controller as defined above.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Examples of the invention are now described with reference to the accompanying drawings, in which:

[0030] Figure 1 schematically illustrates a wind turbine in accordance with an aspect of the invention;

[0031] Figure 2 illustrates a baseline minimum pitch curve according to which the wind turbine of Figure 1 is controlled;

[0032] Figure 3 shows the steps of a method, in accordance with an aspect of the invention, for controlling the wind turbine of Figure 1 according to a modified minimum pitch curve, modified relative to the baseline minimum pitch curve of Figure 2;

[0033] Figure 4 illustrates the baseline minimum pitch curve of Figure 2, the modified minimum pitch curve of Figure 3, and a plurality of further minimum pitch curves each offset from the baseline minimum pitch curve by a respective value of collective pitch angle;

[0034] Figure 5 illustrates estimated angle of attack, of rotor blades of the wind turbine of Figure 1 , against tip-speed ratio for the baseline minimum pitch curve of Figure 2 and each of the further minimum pitch curves of Figure 4; and

[0035] Figure 6 illustrates a proportion of time the wind turbine of Figure 1 spends in stall, for different wind speeds, when controlled in accordance with the baseline minimum pitch curve of Figure 2 and two distinct modified minimum pitch curves each determined through the method of Figure 3.

[0036] DETAILED DESCRIPTION

[0037] 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 (not shown) housed inside the nacelle 103. In addition to the generator, the nacelle 103 houses other components required for converting wind energy into electrical energy, e.g. a gearbox, 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.

[0038] The wind turbine 10 may be controlled in dependence on a variety of conditions, such as wind speed, wind turbulence, wind shear, etc. Therefore, one or more conditions related to the environment in the vicinity of the wind turbine 10 may be determined by, and / or received at, the wind turbine 10. More specifically, wind speed may be determined by, and / or received at, the wind turbine 10. The wind speed may be determined through blade load measurements, an anemometer, and / or any other suitable method known to the skilled person. Moreover, it is noted that the wind speed in the vicinity of the wind turbine 10 may be determined at the wind turbine 10 or determined at a location external to the wind turbine 10 and communicated to the wind turbine 10 via any suitable means.

[0039] The wind turbine 10 may be operated based on an estimated or measured wind speed by controlling operation of the wind turbine 10 based on a power curve, as is well known in the art. Between a cut-in wind speed and a rated wind speed (partial load region), the wind turbine 10 is typically operated by a partial load controller. The partial load region typically includes a variable speed partial load region, between the cut-in wind speed and a wind speed associated with a rated rotor speed, and a constant speed partial load region, between the wind speed associated with rated rotor speed and a wind speed associated with a rated generator power (referred to generally as rated wind speed). In the partial load region the wind turbine is operated with constant collective pitch reference. In the variable speed partial load region the wind turbine is operated with variable generator speed, and in the constant speed partial load region the wind turbine is operated with constant (rated) generator speed. In the partial load region, the wind turbine 10 is typically operated to maximise the energy captured from passing wind in order to maximise the power generated by the wind turbine 10. Such a mode of operation may be referred to as unconstrained partial load operation.

[0040] In an unconstrained partial load operation, the wind turbine 10 may be operated based on a minimum pitch curve, or optimal pitch curve, defined for the specific wind turbine 10 to maximise power output. A baseline / default minimum pitch curve 20 is shown in Figure 2 - illustrated as a plurality of data points - and describes a relationship between collective pitch angle of the rotor blades 106 and a tip-speed ratio (TSR) of the wind turbine 10 to maximise wind turbine power output. The TSR is the ratio between the angular velocity at the tip of a rotor blade 106 and a current wind speed. The baseline minimum pitch curve 20 illustrates the optimal collective pitch angle for a given TSR. Typically, there exists a TSR value which maximises the energy capture achieved by the wind turbine 10, hereinafter referred to as an optimal TSR 202. For example, the optimal TSR may be in the range 8 to 12, and will vary for different wind turbines.

[0041] The combination of the TSR and the collective pitch value may be used to calculate an angle of attack (AoA). The AoA may be defined as an angle between an apparent direction of the wind in the vicinity of the wind turbine and a chord line of a rotor blade 106 of the wind turbine 10. The apparent wind direction is defined as the wind direction measured in the reference frame of the rotor blades 106, i.e. the sum of the incoming wind and an airflow experienced at the rotor blades 106 due to the rotational motion of the rotor blades 106. The AoA may depend on a number of parameters. For instance, the AoA may be estimated based on wind turbine rotational speed, rotor blade pitch angle, wind speed, a radius of the wind turbine rotor 104 and a thrust coefficient Ct(which may be obtained via a look-up table).

[0042] The AoA may be estimated at a specific location on the rotor blade 10, i.e. at a specific point along the length of the rotor blade 106. The specific point may be selected as desired, but in some examples may be at a point in the region of the blade that experiences a significant amount, or most, of the torque / forces produced at the wind turbine rotor 104. For instance, in some examples the specific point may be at least 75% of the distance long the rotor blade 106 from the rotor 104 to the tip of the blade 106. However, it will be understood that this is for illustrative purposes only, and that other points on the rotor blade 106 may be used. One way in which AoA may be estimated is according to

[0043] AoA = D<p — 0col+ where ecolis collective pitch angle, f is a twist correction, D is a constant, and <p may be obtained according to where v is wind speed, is the specific point / distance along the rotor blade, is the wind turbine rotational (rotor) speed, and a is the induction factor and may be expressed as where Ctis the thrust coefficient. It will be understood that AoA may be estimated in different ways.

[0044] For varying AoA, the ratio of lift to drag experienced at the wind turbine rotor blades 106 will change. The rotor blades 106 are designed to operate at a target AoA, which optimises the ratio between the thrust coefficient Ctand a drag coefficient Cd. The target AoA may correspond to the optimal TSR. In a corresponding manner to that mentioned above for the AoA, both the thrust coefficient Ctand drag coefficient Cdmay be for a point along the length of the rotor blades 106 that experiences a significant amount, or most, of the torque / forces.

[0045] There exists a stall point at which the AoA of the rotor blades 106 is such that the ratio between the thrust coefficient Ctand drag coefficient Cdfalls below a threshold value and the wind turbine 10 stalls. When operated in line with the baseline minimum pitch curve 20 of Figure 2, there exists a range of TSR values - generally indicated around the minimum point of the baseline minimum pitch curve 20 by reference numeral 204 - at which the difference between the AoA of the rotor blades 106 and the stall point will be insufficient such that the likelihood of the wind turbine 10 stalling is greater than an acceptable level. These TSR (stall) values 204 may be less than the optimal TSR 202. When the wind turbine 10 operates on the minimum pitch curve 20 near to the optimal TSR 202, there is a sufficient margin to the stall point.

[0046] During constant speed partial load operation, the wind turbine 10 may be operated according to the baseline minimum pitch curve 20. Moreover, because the angular velocity of the rotor blades 106 remains constant in constant speed partial load operation, TSR will decrease with increasing wind speed, and the AoA shifts from the target AoA towards the stall point. When the wind turbine 10 operates in a derated mode of operation, such as in a noise reduction mode, the TSR is reduced and there is a risk that the margin to stall would become insufficient, as explained below.

[0047] Noise is generated as a result of rotation of the rotor blades 106 and of movement of other mechanical components of the wind turbine 10. Generated noise may be greater or more volatile when the wind turbine operates in turbulent wind conditions. Restrictions may be placed on the level of noise that wind turbines 10 are permitted to emit, particularly in cases where the wind turbine 10 is located close to built-up areas such as towns and cities.

[0048] The wind turbine 10 may be operated in accordance with a noise reduction mode or noise control mode to ensure that the emitted noise is no greater than permitted levels. In a noise control mode, the wind turbine 10 may operate with the baseline minimum pitch curve 20. In the noise control mode, the wind turbine 10 may be controlled to limit the rotational velocity of the rotor blades 106, thus reducing the TSR such that the AoA of the rotor blades 106 approaches the stall point. For this reason, consideration of wind turbines stalling is of particular importance when controlling a wind turbine 10 in accordance with a noise control mode during constant speed partial load operation.

[0049] The present invention is advantageous in that operation of a wind turbine to maximise power in a noise control mode can be performed in a manner that ensures a sufficient margin between the AoA of the rotor blades 106 and the stall point is maintained, thereby reducing stall risk. This is achieved by operating the wind turbine according to a minimum pitch curve that has been modified from a baseline minimum pitch curve in a manner that ensures a sufficient margin to stall for all values of TSR. While such a margin to stall could also be achieved by updating values in a noise control mode pitch table, each different noise control mode would need to be tuned separately, which would be a time consuming and laborious process, particularly for a wind turbine with several defined noise control modes.

[0050] Figure 3 shows the steps of a method 30, performed by a controller I control system of the wind turbine 10, for controlling the wind turbine 10 when it is operating in a noise control mode, in accordance with examples of the invention. At step 302 of the method 30, the baseline minimum pitch curve 20 is retrieved. As described above, the baseline minimum pitch curve 20 describes the (minimum) collective pitch angle of the rotor blades 106 of the wind turbine 10 as a function of the TSR to maximise the power output of the wind turbine 10. In some examples, the baseline minimum pitch curve 20 may be determined remotely from the wind turbine 10 and received by the wind turbine 10. Alternatively, the baseline minimum pitch curve 20 may be determined / defined at the wind turbine 10. As such, the baseline minimum pitch curve 20 may be retrieved from a local database at the wind turbine 10, or from a storage location remote from the wind turbine 10, via suitable communication means.

[0051] At step 304, the method 30 involves retrieving a defined stall angle that is an AoA of the wind turbine 10 for stalling the wind turbine 10. In a corresponding manner to above, the defined stall angle may be retrieved from a local database at the wind turbine 10 or may be received from a storage location remote from the wind turbine 10. The defined stall angle may be specific to the specific wind turbine 10 under consideration. The stall angle may be determined via experimentation or simulation, for instance, to determine an AoA that leads to a loss of lift.

[0052] At step 306, the method 30 involves defining a maximum angle of attack that is a defined margin less than the defined stall angle. It is desired to control operation of the wind turbine such that the maximum AoA is not exceeded. This is to ensure that the stall risk of the wind turbine 10 remains at an acceptable level. Indeed, the margin between the maximum AoA and the stall angle may be defined based on what is deemed to be an acceptable stall risk for the wind turbine 10. That is, a larger margin may be defined - leading to a smaller maximum AoA - in cases where a relatively low stall risk is desired, or in cases in which wind volatility increases the risk of operation with insufficient margin to stall. The defined margin can therefore be any suitable value, e.g. any suitable number of degrees.

[0053] At step 308, the method 30 involves determining a modified minimum pitch curve that modifies the baseline minimum pitch curve 20 to maximise power output of the wind turbine 10 against a constraint that the defined maximum AoA is not exceeded. Expressed differently, the baseline minimum pitch curve 20 is modified to ensure that, when the wind turbine 10 is operated according to the (modified) minimum pitch curve, a sufficient margin to stall - i.e. at least the defined margin - is maintained. In the described example, the modified minimum pitch curve is determined using simulations of wind turbine operation according to different minimum pitch curves. The AoA of the rotor blades 106 is monitored during the simulations - in particular, to determine whether the maximum AoA is exceeded - and this output from the simulations is used to modify the baseline minimum pitch curve 20 to ensure that the maximum AoA is not exceeded. This is described in greater detail below.

[0054] The baseline minimum pitch curve 20 is used as the starting point. The different minimum pitch curves to be used as part of the simulations may be referred to as simulation minimum pitch curves. A first simulation minimum pitch curve may be obtained by offsetting the collective pitch angle of the baseline minimum pitch curve 20 by a positive value, e.g. one degree. That is, the first simulation minimum pitch curve may be obtained by shifting the baseline minimum pitch curve 20 upwards by a specific value.

[0055] Figure 4 schematically illustrates the baseline minimum pitch curve 20 and a plurality of simulation pitch curves 401 , 402, 403, 404 in the described example. The first simulation minimum pitch curve 401 is the baseline minimum pitch curve 20 offset by a (constant) positive value of collective pitch angle all of the way along the curve. The second, third and fourth simulation pitch curves 402, 403, 404 are then offset by respective different (constant) positive values of collective pitch angle. A positive offset is an offset of the pitch angle towards a feathered pitch angle.

[0056] The simulation of wind turbine operation may be executed to ensure that the wind turbine 10 operates according to the first simulation minimum pitch curve 401. This may involve simulating operation of the wind turbine 10 in a derated mode of operation, e.g. a noise control mode, and in relatively benign, e.g. turbulence free, wind conditions.

[0057] Operation of the wind turbine 10 is simulated for a range of wind speeds and in a manner that ensures that operation is executed for a range of different TSR values, i.e. for a range of points along the first simulation minimum pitch curve 401 .

[0058] For each wind speed at which the simulation is executed, a pair of collective pitch angle and AoA values is output. The output values may be average values of the relevant parameter over a certain period of time operating at a given wind speed in the simulation. For instance, the collective pitch angle and AoA values may be ten-minute average values of these parameters. The collective pitch angle is determined according to the first simulation minimum pitch curve 401 for a given wind speed and a power curve according to which the wind turbine 10 operation is being simulated. The AoA may be estimated based on the collective pitch angle, e.g. according to the equation defined above for estimated AoA.

[0059] Further simulations are executed to operate the wind turbine 10 according to each of the respective second, third and fourth simulation minimum pitch curves 402, 403, 404, with the (time averaged) collective pitch angle and AoA values being obtained as a function of wind speed, as described above.

[0060] Figure 5 schematically illustrates plots of estimated AoA against TSR obtained from simulation outputs when the wind turbine 10 is operated according to different minimum pitch curves. In particular, Figure 5 shows a baseline AoA plot 50 of estimated AoA values as a function of TSR when the wind turbine 10 is operated according to the baseline minimum pitch curve 20. First, second, third and fourth AoA plots 511 , 512, 513, 514 show estimated AoA values as a function of TSR when the wind turbine 10 is operated according to the respective first, second, third and fourth simulation minimum pitch curves 401 , 402, 403, 404.

[0061] Figure 5 also shows the defined maximum AoA 52 in the described example. It is seen that in the described example, the estimated AoA exceeds the defined maximum AoA 52 for a specific range of TSR values when the wind turbine 10 is operated according to the baseline minimum pitch curve 20, i.e. estimated AoA on the baseline AoA curve 50 exceeds the defined maximum AoA 52 for the specific range of TSR values.

[0062] When the wind turbine 10 is operated according to the first simulation minimum pitch curve 401 , the estimated AoA - as shown in the first AoA plot 51 1 - also exceeds the defined maximum AoA 52 for certain TSR values. However, the range of TSR values that the first AoA plot 511 exceeds the defined maximum AoA 52 is smaller than that for the baseline AoA plot 50. Indeed, this range of TSR values for the first AoA plot 51 1 is a sub-range of that for the baseline AoA plot 50. Furthermore, the degree to which the values on the baseline AoA plot 50 exceed the defined maximum AoA 52 is greater than the degree to which the values on the first AoA plot 511 exceed the defined maximum AoA 52. It is noted that the estimated AoA values on each of the second, third and fourth AoA plots 512, 513, 514 are less than the defined maximum AoA 52 for all values of TSR. The curves / plots illustrated in Figures 4 and 5 may be stored as lookup tables of values for each different minimum pitch curve. In particular, the pairs of collective pitch angle and estimated AoA output from the simulation for TSR value may be stored in a lookup table for each different minimum pitch curve.

[0063] In an alternative embodiment, the control system may implement an AoA estimator and an integral controller. The AoA error to the desired AoA limit may be integrated to form a pitch offset. It may, in this embodiment, be necessary to apply integrator clamping to prevent activation outside of the relevant TSR interval.

[0064] It is desired to maximise the AoA while still maintaining the defined margin to stall, i.e. while not exceeding the defined maximum AoA 52. From the plots in Figures 4 and 5 - or, equivalently, the lookup tables outlined above - a map of pairs of values of collective pitch angle and AoA for all of the different minimum pitch curves may be determined. An interpolation of these mapped points may be performed to obtain the collective pitch angle that maximises AoA without the AoA exceeding the defined maximum AoA, i.e. constraining AoA to maintain a sufficient margin to stall.

[0065] Referring again to step 308 of Figure 3, the interpolated collective pitch angle plot / values may be used to modify the baseline minimum pitch curve 20 to obtain a (modified) minimum pitch curve that, when the wind turbine 10 is operated according to the same, does not result in the defined maximum AoA 52 being exceeded, while still seeking to maximise power output.

[0066] Figure 4 shows the modified minimum pitch curve 42 in the described example. It is seen that the modified minimum pitch curve 42 follows the baseline minimum pitch curve 20 for certain values of TSR, namely, for values of TSR where estimated AoA is less than the defined maximum AoA 52 (see Figure 5). Specifically, the modified minimum pitch curve 42 follows the baseline minimum pitch curve 20 for TSR values up to a lower bound 431 , and greater than an upper bound 432, of a specific range 44 of TSR values. For TSR values within the specific range 44, the collective pitch angle of the baseline minimum pitch curve 20 is modified to be the interpolated collective pitch angle obtained as described above. In this way, operation according to the modified minimum pitch curve 42 ensures that a sufficient margin to stall is maintained. In some examples, prior to determining the modified minimum pitch curve 42, it may be defined for which values of TSR the baseline minimum pitch curve 20 is to be modified. The baseline minimum pitch curve 20 is then modified only for these values. The TSR values may be a specific range, such as the defined range 44. The specific range 44 may be defined to include only those TSR values that correspond to collective pitch angle values, on the baseline minimum pitch curve 20, less than a defined threshold value. The specific range 44 may be defined to exclude certain values of TSR, such as an optimal TSR point 202 of the wind turbine 10, as it may be desired to operate the wind turbine 10 to maximise power output at this operating point. Indeed, the specific range 44 of TSR values to be modified may generally be a range of values less than the optimal TSR point 202. In order to determine the modified minimum pitch curve 42, an interpolation may be performed at each end point of the specific range 44 to ensure a relatively smooth transition away from the baseline curve 20 onto the modified portion of the modified minimum pitch curve 42 (in the specific range 44).

[0067] Referring again to Figure 3, at step 310 the method 30 involves controlling the wind turbine 10 to operate in the noise control mode according to the modified minimum pitch curve 42. This may involve adjusting a collective blade pitch reference or an output power reference of the wind turbine 10. As described above, the AoA may be at particular risk of breaching the desired margin to stall when the wind turbine 10 is operating in a noise control mode in the constant speed partial load region of the power curve. As such, controlling the wind turbine 10 in accordance with the modified minimum pitch curve 42 may be of particular benefit when in the constant speed partial load region. In some examples, operation of the wind turbine 10 according to the modified minimum pitch curve 42 may even be limited to when it is in, or near to, the constant speed partial load region. For instance, this could be implemented as utilising the modified minimum pitch curve 42 only for certain wind speeds, and using the baseline curve 20 for other wind speeds.

[0068] In some examples, the maximum AoA is defined based on a determined wind condition in which the wind turbine is operating. For instance, the maximum AoA may be determined to be lower in non-benign wind conditions, e.g. high turbulence or high wind shear conditions, relative to in relatively benign conditions. The AoA can vary by a greater amount in turbulent conditions, meaning that operation of the wind turbine 10 near to the maximum AoA increases the amount of time the wind turbine 10 may operate with a margin to stall lower than is desired. The maximum AoA may therefore be lowered, e.g. from a default value, in such wind conditions to reduce the risk of stall in such conditions. In examples of variable maximum AoA, the described method may therefore involve monitoring wind turbine sensor data indicative of the wind conditions in which the wind turbine 10 is operating, and then changing / modifying the maximum AoA based on the monitored sensor data. The sensor data may for instance be measured data from blade load sensors of the wind turbine 10 indicating the loading experienced by the wind turbine 10, and then determining whether the wind turbine 10 is operating in benign or non-benign conditions based on the loading data.

[0069] The method 30 may involve determining a plurality of different modified minimum pitch curves for a respective plurality of different defined maximum AoA values, where each modified minimum pitch curve maximises power output when constrained by the relevant maximum AoA value. When the defined maximum AoA is changed, e.g. based on current wind conditions, the method 30 may involve switching from one of the modified minimum pitch curves to another one based on the updated maximum AoA. In other examples, when the defined maximum AoA is changed the method 30 may involve determining a new modified minimum pitch curve according to the method steps described above.

[0070] Figure 6 schematically illustrates plots 62, 64, 66 showing a proportion of time spent in stall for a range of wind speeds when the wind turbine 10 is operated according to different minimum pitch curves. A baseline stall curve 62 shows time spent in stall when operating according to the baseline minimum pitch curve 20. First and second stall curves 64, 66 shows time spent in stall when operating according first and second modified minimum pitch curves, determined using the method described above. Advantageously, it can be seen that the proportion of time for which the wind turbine 10 is stalled decreases when the wind turbine 10 is operated in accordance with the first and second modified minimum pitch curves relative to operation of the wind turbine 10 in accordance with the baseline minimum pitch curve 20.

[0071] The first stall curve 64 shows a lower proportion of time in stall than the second stall curve 66. The difference in the proportion in stall between the first and second stall curves 64, 66 is due to the wind turbine 10 being controlled in accordance with different modified minimum pitch curves. The first stall curve 64 corresponds to operation of the wind turbine 10 in accordance with a first modified pitch curve that has a larger defined margin to stall than a second modified pitch curve corresponding to the second stall curve 66. A controller of the wind turbine 10 for performing the described method 30 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. The controller may be located in one or more locations of the wind turbine 10, e.g. in the wind turbine tower 102.

[0072] Many modifications may be made to the described examples without departing from the scope of the appended claims.

[0073] In the described examples, the wind turbine 10 is operated according to a modified minimum pitch angle when operating in a noise control mode. More generally, in examples of the invention the wind turbine 10 may be operated according to a modified minimum pitch angle when operating in other derated modes of operation such that the margin to stall of the AoA risks becoming insufficient. Indeed, the wind turbine 10 may be operated according to a minimum pitch curve in any suitable mode of wind turbine operation, but finds particular benefit in those operational modes in which the margin to stall of the AoA is at risk of becoming too small.

[0074] In the described examples, operation of the wind turbine 10 according to various minimum pitch curves is tested / executed via simulation. However, it will be understood that in different examples field testing may be performed to analyse wind turbine operation according to different minimum pitch curves.

[0075] In the described examples, the different minimum pitch curves used to analyse wind turbine performance are defined as an offset (in collective pitch angle) of a baseline minimum pitch curve 20. However, it will be understood that these minimum pitch curves may be defined in any other suitable manner, and may not be defined as constant offsets to a baseline curve.

Claims

CLAIMS1 . A method of controlling a wind turbine that is operating in a noise control mode, the method comprising: retrieving a baseline minimum 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, to maximise power output of the wind turbine; retrieving a defined stall angle that is an angle of attack of the wind turbine for stalling the wind turbine; defining a maximum angle of attack that is a defined margin less than the defined stall angle; determining a modified minimum pitch curve that modifies the baseline minimum pitch curve to maximise power output of the wind turbine against a constraint that the defined maximum angle of attack is not exceeded; and controlling the wind turbine to operate in the noise control mode according to the modified minimum pitch curve.

2. A method according to Claim 1 , wherein determining the modified minimum pitch curve comprises modifying the baseline minimum pitch curve only for TSR values within a defined TSR range of TSR values.

3. A method according to Claim 2, wherein the defined TSR range does not include a defined optimal TSR value of the wind turbine; optionally, wherein the defined TSR range includes only TSR values less than the defined optimal TSR value.

4. A method according to any previous claim, the method comprising controlling the wind turbine to operate according to the modified minimum pitch curve in a constant speed partial load region of a defined power curve of the wind turbine.

5. A method according to any previous claim, wherein determining the modified minimum pitch curve comprises: executing a plurality of wind turbine simulations in which the wind turbine is operated in the noise control mode, wherein executing each wind turbine simulation comprises:defining a respective simulation minimum pitch curve describing collective pitch angle as a function of TSR of the wind turbine; controlling the wind turbine to operate according to the respective simulation minimum pitch curve for different wind speeds; obtaining a pair of collective pitch angle and angle of attack values for each of a plurality of wind speeds as output from the wind turbine simulation; based on the obtained pair of values and the simulation minimum pitch curve of each of the plurality of wind turbine simulations, determining angle of attack as a function of TSR to maximise angle of attack against the constraint that the defined maximum angle of attack is not exceeded; and determining collective pitch angle as a function of TSR based on the determined angle of attack as a function of TSR to determine the modified minimum pitch curve.

6. A method according to Claim 5, wherein defining the respective simulation minimum pitch curve comprises offsetting collective pitch angle of the baseline minimum pitch curve by a respective positive value.

7. A method according to Claim 5 or Claim 6, wherein determining collective pitch angle comprises: determining a map of the obtained pairs of collective pitch angle and angle of attack values for each of the wind turbine simulations; interpolating the collective pitch angle values in the determined map to maximise angle of attack against the constraint that the defined maximum angle of attack is not exceeded, wherein the modified minimum pitch curve is determined based on the interpolated collective pitch angle values.

8. A method according to Claim 7 when dependent on Claim 2, wherein the defined TSR range includes a defined maximum TSR value, the method comprising interpolating the modified minimum pitch curve at TSR values less than the defined maximum TSR value with the baseline minimum pitch curve at TSR values greater than the defined maximum TSR value to determine the modified minimum pitch curve.

9. A method according to any previous claim, wherein the maximum angle of attack is defined based on a determined wind condition in which the wind turbine is operating.

10. A method according to Claim 9, the method comprising: receiving sensor data, from one or more sensors of the wind turbine, indicative of loading on one or more components of the wind turbine; optionally, wherein the received sensor data is indicative of tilt / yaw moment on a rotor of the wind turbine; and determining the wind condition in which the wind turbine is operating based on the received sensor data.1 1. A method according to Claim 9 or Claim 10, wherein when the wind condition is determined to be a benign wind condition then the method comprises controlling the wind turbine to operate in accordance with the baseline minimum pitch curve.

12. A method according to any of Claims 9 to 11 , wherein when the wind condition is a non-benign wind condition the maximum angle of attack is defined to be less than the maximum angle of attack when the wind condition is a benign wind condition.

13. A method according to any previous claim, wherein controlling the wind turbine comprises adjusting a blade pitch reference or an output power reference of the wind turbine.

14. A controller for controlling a wind turbine that is operating in a noise control mode, the controller being configured to: retrieve a baseline minimum 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, to maximise power output of the wind turbine; retrieve a defined stall angle that is an angle of attack of the wind turbine for stalling the wind turbine; define a maximum angle of attack that is a defined margin less than the defined stall angle; determine a modified minimum pitch curve that modifies the baseline minimum pitch curve to maximise power output of the wind turbine against a constraint that the defined maximum angle of attack is not exceeded; and control the wind turbine to operate in the noise control mode according to the modified minimum pitch curve.

15. A wind turbine comprising a controller according to Claim 14.

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