Wind turbine control in accordance with a maximum allowable noise level

Abstract

The invention relates to controlling a wind turbine in accordance with a maximum allowable noise level. The invention includes defining a noise function describing a relationship between wind turbine rotational speed and noise level generated by the wind turbine. The invention includes defining the maximum allowable noise level, and setting the noise level in the noise function to be equal to the defined maximum allowable noise level. The invention includes solving the noise function for the rotational speed, with the noise level equal to the defined maximum allowable noise level, to obtain a maximum allowable rotational speed of the wind turbine. The invention includes controlling the wind turbine to operate in accordance with the maximum allowable rotational speed.

Description

[0001] WIND TURBINE CONTROL IN ACCORDANCE WITH A MAXIMUM ALLOWABLE

[0002] NOISE LEVEL

[0003] TECHNICAL FIELD

[0004] The invention relates to controlling a wind turbine in accordance with a maximum allowable noise level. In particular, the wind turbine is controlled in accordance with a maximum allowable rotational speed determined using the maximum allowable noise level and a defined noise function describing a relationship between noise level and rotational speed.

[0005] BACKGROUND

[0006] Wind turbines are used to capture energy in the wind as it flows past them, and 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] Wind turbines generate and output noise / sound as they operate. For instance, noise may be generated as a result of rotating rotor blades of the wind turbine encountering turbulent wind in the vicinity of the wind turbine. As another example, noise may be generated by moving mechanical components of the wind turbine.

[0008] Typically, restrictions are placed on the level of noise that a wind turbine is permitted to emit, particularly in cases where the wind turbine is located close to built-up areas such as towns and cities. Such restrictions may be defined according to regulations set for a given territory, e.g. specific country, district, etc., and may vary for different territories. Furthermore, maximum allowable / permitted noise levels generated by a wind turbine are typically a function of wind speed in the vicinity of the wind turbine. Specifically, maximum allowable noise level tends to be lower at lower wind speeds. This is because noise generated by a wind turbine may be heard more clearly at lower wind speeds, whereas at higher wind speeds wind turbine noise may be less audible amongst other generated sound.

[0009] Wind turbines are operated to ensure they do not breach defined noise level restrictions. However, adjusting wind turbine operation to reduce emitted noise typically reduces the level of electrical power being generated by the wind turbine, i.e. it reduces wind turbine operating efficiency. As such, controlling a wind turbine to maximise operating efficiency while remaining within permitted noise levels typically means the wind turbine is operated with the generated noise being as close to the maximum allowable noise level as possible.

[0010] Controlling a wind turbine in this manner may be performed using noise / sound level curves defined as an allowable noise (decibel) level for each wind speed, i.e. as a function of wind speed. Each defined curve may be referred to as a noise mode. For some noise modes, the noise / sound level may change for each wind speed, whereas for other noise modes the noise level may be constant. A wind turbine may have several noise modes associated therewith.

[0011] Typically, the rotational speed and / or rotor blade pitch angle of the wind turbine may be adjusted to control the wind turbine in accordance with a defined noise mode. This means that each noise mode needs to be tuned by adjusting parameters for rotational speed and rotor blade pitch angle. This can be a time consuming and laborious process, particularly for a wind turbine with several defined noise modes.

[0012] It is against this background to which the present invention is set.

[0013] SUMMARY OF THE INVENTION

[0014] According to an aspect of the invention there is provided a method of controlling a wind turbine in accordance with a maximum allowable noise level of the wind turbine. The method comprises defining a noise function describing a relationship between rotational speed of the wind turbine and noise level generated by the wind turbine. The method comprises defining the maximum allowable noise level, and setting the noise level in the noise function to be equal to the defined maximum allowable noise level. The method comprises solving the noise function for the rotational speed, with the noise level equal to the defined maximum allowable noise level, to obtain a maximum allowable rotational speed of the wind turbine. The method comprises controlling the wind turbine to operate in accordance with the maximum allowable rotational speed.

[0015] The noise function may be defined as an equation describing noise level generated by the wind turbine as a function of rotational speed of the wind turbine. Solving the noise function for the rotational speed may comprise: inverting the noise function to obtain an inverse noise function that is an equation describing rotational speed of the wind turbine as a function of noise level generated by the wind turbine; and solving the inverse noise function for the rotational speed, with the noise level equal to the defined maximum allowable noise level, to obtain the maximum allowable rotational speed of the wind turbine.

[0016] The relationship between rotational speed and noise level in the noise equation may be dependent on an angle of attack of the wind turbine, the method comprising setting the angle of attack in the noise function to be equal to an angle of attack value, wherein the noise function is solved for the rotational speed with the angle of attack equal to the angle of attack value; optionally, wherein the angle of attack is angle of attack at a defined distance along the length of rotor blades of the wind turbine.

[0017] The angle of attack value is an estimated angle of attack, the method comprising determining the estimated angle of attack based on wind turbine rotational speed, rotor pitch angle of one or more rotor blades of the wind turbine, wind speed in the vicinity of the wind turbine, a coefficient of thrust of the wind turbine and a radius of a rotor of the wind turbine.

[0018] Determining the estimated angle of attack may comprise including a correction term to account for twist of the rotor blades at the defined distance along the length of the rotor blades.

[0019] The rotor pitch angle may be a rotor blade pitch reference for controlling the rotor blades. The rotor blade pitch reference may be set to be greater than a threshold pitch angle below which the wind turbine will stall.

[0020] Defining the maximum allowable noise level may comprise: obtaining a wind speed signal indicative of wind speed in the vicinity of the wind turbine; and based on the obtained wind speed, retrieving the maximum allowable noise level from a lookup table comprising pairs of wind speed and corresponding maximum allowable noise level values.

[0021] The method may comprise selecting a noise mode from a plurality of defined noise modes. Each defined noise mode may define a relationship describing maximum allowable noise level as a function of wind speed. Defining the maximum allowable noise level may comprise defining the maximum allowable noise level in accordance with the selected noise mode. The method may comprise monitoring a measured noise level, using an acoustic sensor, generated by the wind turbine when the wind turbine is being operated in accordance with the maximum allowable rotational speed. The method may comprise determining an error between the measured noise level and the defined maximum allowable noise level. The method may comprise adjusting the maximum allowable rotational speed to reduce the determined error.

[0022] The method may comprise a training phase. In the training phase, the method may comprise method steps of, at a plurality of different wind speeds: monitoring measured noise level when the wind turbine is being operated in accordance with the maximum allowable rotational speed, obtained from the noise function, for the respective wind speed; determining a difference between measured noise level and maximum allowable noise level for the respective wind speed; determining a correction value to the maximum allowable rotational speed to reduce the determined difference; and fitting a correction curve using the correction values and corresponding wind speeds as interpolation points. The method may further comprise, in an execution phase, controlling the wind turbine to operate in accordance with an updated maximum allowable rotational speed obtained by adjusting the maximum allowable rotational speed, obtained from the noise function, according to a current correction value obtained from the correction curve.

[0023] A different correction curve may be fitted for each different defined noise mode of the wind turbine or each of one or more defined environmental conditions in which the wind turbine operates.

[0024] Controlling the wind turbine to operate in accordance with the maximum allowable rotational speed may comprise determining one or both of rotor blade pitch reference and output power reference of the wind turbine based on a wind turbine rotational speed reference. The wind turbine rotational speed reference may be set to be no greater than the maximum allowable rotational speed. Controlling the wind turbine may then comprise adjusting rotor blade pitch and / or output power of the wind turbine in accordance with the respective rotor blade pitch reference and / or output power reference.

[0025] If the wind turbine rotational speed reference is less than the maximum allowable rotational speed, then it may be that no adjustment of the wind turbine rotational speed reference is performed. Controlling the wind turbine to operate in accordance with the maximum allowable rotational speed may comprise adjusting one or both of rotor blade pitch reference and output power reference of the wind turbine so that wind turbine rotational speed remains no greater than the maximum allowable rotational speed.

[0026] If wind turbine rotational speed is less than the maximum allowable rotational speed, then the method may comprise performing no adjustment to one or both of rotor blade pitch reference and output power reference of the wind turbine.

[0027] 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.

[0028] According to another aspect of the invention there is provided a controller for controlling a wind turbine in accordance with a maximum allowable noise level of the wind turbine. The controller is configured to define a noise function describing a relationship between rotational speed of the wind turbine and noise level generated by the wind turbine. The controller is configured to define the maximum allowable noise level, and set the noise level in the noise function to be equal to the defined maximum allowable noise level. The controller is configured to solve the noise function for the rotational speed, with the noise level equal to the defined maximum allowable noise level, to obtain a maximum allowable rotational speed of the wind turbine. The controller is configured to control the wind turbine to operate in accordance with the maximum allowable rotational speed.

[0029] According to another aspect of the invention there is provided a wind turbine comprising a controller as defined above.

[0030] BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Examples of the invention are now described with reference to the accompanying drawings, in which:

[0032] Figure 1 schematically illustrates a wind turbine in accordance with an aspect of the invention; Figure 2 schematically illustrates a controller of the wind turbine of Figure 1 in accordance with an aspect of the invention; and

[0033] Figure 3 shows the steps of a method performed by the controller of Figure 2 in accordance with an aspect of the invention.

[0034] DETAILED DESCRIPTION

[0035] 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, 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.

[0036] Figure 2 schematically illustrates an example of a control system or overall controller 20 of the wind turbine 10 that includes a (feedback) speed controller or control block / module 202 implemented to determine collective pitch actuation signals for controlling pitch of the rotor blades 106. This may be a full load controller (FLC) or partial load controller (PLC) of the wind turbine 10. In the illustrated implementation, the speed controller 202 minimises a speed error (speedrot - speedy) between the actual rotor speed (speedrot) and a reference rotor speed (speedy) in order to output a requested power P (in the form of a power setpoint) and a collective pitch reference, 0COi. The collective pitch reference as determined by the speed controller 202, in view of the rotor speed, may also take further sensor values into account. This is referred to in Figure 2 as a measurement set, ms, being input into the speed controller 202. In some examples, the collective pitch reference may be modified / adjusted according to offset values / signals output by an individual pitch controller (not shown in Figure 2) of the wind turbine 10.

[0037] The feedback speed controller 202 may be implemented by a PI (proportional-integral), PID (proportional-integral-derivative), or similar control scheme. In one example, the speed controller 202 may alternatively be a model predictive controller which, based on minimising a cost function, is arranged to determine the collective pitch reference and / or the power reference. The reference rotor speed (speeds) is set / determined by a reference rotor speed block / module 204 and input into the speed controller 202. The reference rotor speed may be set based on various factors / parameters, e.g. wind speed. In the described example, the reference rotor speed is set based on a maximum allowable / permitted rotor speed (speedmax) that is determined by a maximum rotor speed block / module 206 and then input into the reference rotor speed module 204. The determinations performed by these modules 204, 206 will be described in greater detail below.

[0038] The present invention is directed to determining a maximum allowable rotor speed of a wind turbine that, when used to control wind turbine operation, ensures that a maximum noise level that is permitted / allowed to be generated / output by the wind turbine according to a defined noise mode is not exceeded. The present invention is advantageous in that minimal or zero tuning of the defined noise mode is needed in order to determine the maximum allowable rotor speed, meaning the invention provides a simpler, faster and less laborious approach than some known approaches. The benefits of the present invention are achieved by virtue of the fact that a defined (inverted) equation describing wind turbine noise level as a function of wind turbine rotor / rotational speed is used to determine maximum allowable rotor speed. This is described in greater detail below. Further beneficial effects of the invention will become apparent from the below description.

[0039] In some examples, the noise / sound level emitted by the wind turbine 10 may be a sound power level LWAproduced by the wind turbine 10, where LWAis measured in decibels (dB). The noise emitted by a wind turbine may be estimated / determined using a defined equation for sound power as a function of various parameters. In particular, the defined noise equation is a function of wind turbine rotor (rotational) speed. The noise equation may be a function of an angle of attack of the wind turbine. The noise equation may be a function of one or more further tuneable (noise) parameters. The noise equation may provide the sound output of the whole wind turbine 10.

[0040] As a purely illustrative example, a noise equation for sound power level LWAmay be defined as

[0041] ^WA=G ’ |AoA — AoA0| + C2• Vtip where 7tipis wind turbine rotational speed, AoA is angle of attack, AoA0is a minimum angle of attack past which noise emission levels start increasing again for decreasing angle of attack AoA, and , C2are appropriately tuned parameters. In this illustrative example, sound power level LWAincreases for increasing rotational speed 7tip, and sound power level LWAvaries linearly with angle of attack AoA on top of the rotational speed 7tip.

[0042] It will be understood that the present invention is not limited to the illustrative noise equation / function provided above, and that the skilled person will be aware of different noise equations / functions that may be used in conjunction with the methods of the invention provided herein. It will be understood that in different examples - unlike the above example - the noise equation / function may not be able to be expressed in explicit form for sound power level. More generally, a noise equation for use in examples of the present invention may be expressed as

[0043] LWA=f(RotSpd, AoA, NoiseParameters)

[0044] That is, sound power level LWAis a function f of wind turbine rotational speed, e.g. 7tip, angle of attack and one or more noise parameters, such as C1;C2in the above illustrative example of a noise equation.

[0045] In examples of the present invention, the sound power level LWAmay be set to be a maximum allowable / permitted noise level and the defined noise equation may be solved for wind turbine rotational speed to obtain a maximum allowable / permitted rotational speed. Referring back to Figure 2, this determination of the maximum allowable rotational speed is performed by the speedmax module 206.

[0046] In some examples, this may involve inverting the defined function f for sound power level LWAto obtain an inverted equation for rotational speed, i.e.

[0047] RotSpdmax= g LWAmax, AoA, NoiseParameters) where the maximum allowable rotational speed RotSpdmaxis a function g of the maximum allowable sound power level LWAmax. angle of attack and one or more noise parameters. The function g can then be solved / evaluated for RotSpdmax.

[0048] In some examples, solving the noise equation for maximum allowable rotational speed may include the use of one or more numerical methods, e.g. where an explicit expression for RotSpdmacannot be obtained. Such numerical methods will be known to the skilled person, e.g. Newton’s method, bisection method, etc.

[0049] The maximum allowable noise level (sound power level) to be used for solving the (inverted) noise equation may be obtained from a look-up table. In particular, the look-up table may store different maximum allowable noise level values for different wind speeds. These look-up tables may constitute noise / sound level curves defined as an allowable noise (decibel) level for each wind speed, i.e. as a function of wind speed, where each defined curve may be referred to as a noise mode.

[0050] The wind turbine 10 may have a number of noise modes associated therewith, and the noise modes may define allowable noise levels as a function of wind speed according to defined regulations, e.g. for a given territory in which the wind turbine 10 is located. In some examples, the noise modes may be stored in a different form from a look-up table, e.g. as a defined function that can be evaluated / solved for maximum allowable noise level for a given wind speed.

[0051] For these purposes, wind speed can be obtained in any suitable manner known to the skilled person, e.g. measured wind speed using one or more wind speed sensors or estimated wind speed using an online estimator. The wind turbine 10 may be operating according to one specific noise mode, and so the look-up table corresponding to this specific noise mode may be queried to obtain the maximum allowable noise level for the (current) wind speed.

[0052] The maximum allowable rotational speed determined by the speedmax module 206 is provided as input to the speedy module 204, and the determined maximum allowable rotational speed is used as a limitation to the reference rotational speed signal set by the speedref module 204. In particular, as mentioned above a reference rotational speed may be determined / set based on one or more parameters, e.g. wind speed, rotor blade loading, etc.

[0053] In examples of the invention, the speedref module 204 may be configured to compare the determined / set reference rotational speed against the maximum allowable rotational speed determined obtained from the speedmax module 206. If the determined reference rotational speed is less than, or equal to, the maximum allowable rotational speed then the reference rotational speed may remain unaltered / unchanged and provided as input to the speed controller 202. If the determined reference rotational speed is greater than the maximum allowable rotational speed then the reference rotational speed may be reduced / decreased to be equal to the maximum allowable rotational speed (or to be slightly less than the maximum allowable rotational speed). This reduced / decreased reference rotational speed may then be provided as input to the speed controller 202. The maximum allowable rotational speed and reference rotational speed may be determined at each time step (sample-by-sample) of the control system 20 by the respective modules 206, 204. Alternatively, these parameters may be determined at defined time intervals, e.g. every ten seconds, rather than sample-by-sample. This may have an averaging / smoothing effect on the obtained parameter values. The speed controller 202 - taking the reference rotational speed as input - determines pitch position 0coi of the rotor blades 106 and power P such that the maximum allowable rotational speed is not exceeded / surpassed.

[0054] In examples in which the noise equation is dependent on angle of attack, it is apparent that a suitable estimate of angle of attack is needed in order to solve the (inverted) noise equation for maximum allowable rotational speed. The angle of attack estimated for this purpose may be a maximum angle of attack at which it is desired to operate the wind turbine 10. The angle of attack may be defined as an angle between a direction of the wind in the vicinity of the wind turbine (incoming wind direction) and a chord line of a rotor blade 106 of the wind turbine 10.

[0055] The angle of attack may depend on a number of parameters. For instance, the angle of attack 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).

[0056] The angle of attack 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 that experiences a significant amount, or most, of the torque / forces produced by 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 may be used.

[0057] One way in which angle of attack AoA may be estimated is according to

[0058] AoA = D<p — 9C0\ + where ecolis collected 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 may be expressed as l - V(l - ) a 2

[0059] It will be understood that angle of attack may be estimated in different ways.

[0060] It is noted that the noise equation is dependent on (collective) pitch angle as well as rotational speed. However, the rotational speed may be regarded as affecting generated noise levels more than pitch angle, and so it may be regarded as being preferable I more effective to adjust rotational speed (via the determined maximum allowable rotational speed) - rather than pitch angle - to ensure that emitted noise remains within permitted levels.

[0061] The methods described herein may be of particular use when the wind turbine 10 operates in partial load conditions. Wind turbine rotational speed increases with increasing wind speed in partial load operation. Also, the wind turbine 10 operates in partial load at lower wind speeds and the noise emitted by the wind turbine 10 may be more apparent / audible at lower wind speeds. As permitted noise levels are provided as a function of wind speed - typically with lower permitted noise levels at lower wind speeds - then noise reduction strategies may be needed more during partial load operation.

[0062] In examples of the invention described herein, a defined function / equation is used to obtain a maximum allowable rotational speed of the wind turbine that corresponds to a maximum allowable noise level generated by the wind turbine. However, there may in some cases be a discrepancy between the noise level predicted / estimated according to the defined function / equation, and the noise level actually generated by the wind turbine, for a given wind turbine rotational speed. This may be caused by various factors apparent in the field but not captured in the defined function / equation, e.g. environmental conditions, real-world effects.

[0063] Correcting the defined noise function / equation - in particular, correcting / changing the parameters in the noise equation - to fit the equation to measured noise level data obtained in the filed may be time consuming and may require defining many different noise modes to account for the various different factors that lead to discrepancies between the predicted and actual noise levels.

[0064] Instead, in examples of the invention measured noise level data obtained by monitoring noise emitted by a wind turbine during operation may be used to correct the output of the defined noise equation (rather than correct the noise equation itself). The monitoring / test period may be regarded as a training phase, and may be performed over any suitable period of time, e.g. several months or one year. In particular, for a given / monitored wind speed and wind turbine rotational speed, the measured noise level is compared against the estimated noise level from the noise equation. The error / difference between the measured and estimated noise levels is used to determine a correction value to be applied to the wind turbine rotational speed. When the corrected rotational speed is used in the noise equation then the error between the estimated and measured noise level is minimised / reduced. The correction value may be obtained by solving the noise equation for maximum allowable rotational speed using the measured noise level, for instance.

[0065] This may be repeated for various different wind speeds (and associated maximum permitted noise levels / references). This could be repeated for different noise modes and / or different external factors such as environmental conditions. The output may therefore be a look-up table such as in Table 1 shown below in which each pair of wind speed (WindSpd) and estimated noise level (LwA Ref) has a corresponding correction value to be applied to the wind turbine rotational speed obtained by solving the noise equation. An interpolation algorithm may be applied to the correction values, as a function of wind speed, so that a correction value for any wind speed may be obtained by evaluating the interpolated curve at the appropriate point (wind speed).

[0066] Table 1 This approach is beneficial in that only relatively few interpolation points are needed and that it avoids the need for many different noise modes. In any case, the noise equation will typically provide estimated noise levels that are relatively close to the actual noise levels, and so correction via interpolation is appropriate.

[0067] Figure 3 summarises the steps of a method 30 performed by the controller I control system 20 in accordance with examples of the invention. At step 302, the method 30 involves defining a noise equation / function describing a relationship between rotational (rotor) speed of the wind turbine and noise level generated by the wind turbine 10. The noise function may be defined as an equation describing noise level generated by the wind turbine 10 as a function of rotational speed of the wind turbine. In the noise equation, the noise level may be regarded as a sound power level.

[0068] The relationship between rotational speed and noise level in the noise equation may be dependent on an angle of attack of the wind turbine 10. The method 30 may therefore involve estimating angle of attack for use in the noise equation. The angle of attack may be estimated at a specific distance along the length of the rotor blades 106, typically closer to the blade tip than to the rotor 104. Angle of attack may be estimated based on various parameters, including wind speed, turbine rotational speed and rotor blade (collective) pitch angle. Estimating the angle of attack may also involve the use of a correction term to account for blade twist at the specific point of the blade where angle of attack is being determined. The angle of attack used in the noise equation when determining maximum allowable rotational speed may be the maximum angle of attack it is desired / permitted to operate at.

[0069] The rotor blade pitch may be a pitch reference according to which the pitch angle of the rotor blades is controlled. The pitch reference may typically be greater than a threshold reference below which the wind turbine 10 may stall. However, in some cases the wind turbine 10 may be operating with a pitch reference at which there is a risk that the wind turbine 10 may stall. The estimated angle of attack may be used to indicate that the wind turbine is stalling and so increase the emitted sound power.

[0070] Referring back to Figure 3, at step 304 the method 30 involves defining the maximum allowable noise level, and setting the noise level in the noise function defined at step 302 to be equal to the defined maximum allowable noise level. The maximum allowable noise level may typically be defined based on wind speed, e.g. measured or estimated wind speed. The maximum allowable noise level may be obtained from a look-up table - in memory accessible by the controller 20 - that stores pairs of wind speeds and corresponding maximum allowable noise values.

[0071] A plurality of such look-up tables may be associated with the wind turbine 10, with each table corresponding to a different noise mode. The wind turbine 10 may be controlled to operate in one of these noise modes - e.g. based on the geographical location / territory of the wind turbine 10, other environmental conditions, etc. - and the maximum allowable noise level value may then be retrieved from the corresponding look-up table using the obtained wind speed. This determination may be performed at the sampling rate of the controller 20, for instance. Instead of look-up tables, the noise modes may be stored in the form of defined relationships between wind speed and maximum allowable noise level, e.g. as a defined function, in which case the defined relationship / function is solved for maximum allowable noise level when evaluated at the obtained wind speed.

[0072] At step 306, the method 30 involves solving the noise function / equation for the wind turbine rotational speed, with the noise level equal to the defined maximum allowable noise level obtained in step 304, to obtain a maximum allowable rotational speed of the wind turbine 10. In some examples, this may involve inverting the noise function to obtain an inverse noise function that is an equation describing rotational speed of the wind turbine 10 as a function of noise level generated by the wind turbine 10, and then solving the inverted noise function for the maximum allowable rotational speed with the noise level set to the maximum allowable noise level.

[0073] At step 308, the method 30 involves controlling the wind turbine 10 to operate in accordance with the maximum allowable rotational speed. This may be implemented as a limitation on a rotational speed reference that is used by the speed controller 202 to determine collective pitch reference and output power. In particular, if the maximum allowable rotor speed is less than the rotational speed reference, then the rotational speed reference used by the speed controller 202 may remain unchanged. However, if the maximum allowable rotor speed is greater than the rotational speed reference, then the rotational speed reference used by the speed controller 202 may be adjusted / reduced to be no greater than the maximum allowable rotor speed. For instance, the rotational speed reference may be set to be equal to the maximum allowable rotor speed, or a factor may be applied to the maximum allowable rotor speed to obtain a rotational speed reference that is slightly less than the maximum (e.g. for tolerance, to ensure that maximum permitted noise levels are not breached). Such a reduced rotational speed reference will then impact the collective pitch reference and / or output power determined by the speed controller 202.

[0074] A controller 20 of the wind turbine 10 for performing the described method 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.

[0075] Many modifications may be made to the described examples without departing from the scope of the appended claims.

[0076] In the above-described examples, a maximum allowable rotor speed is determined and used to control operation of a wind turbine. More generally, a maximum allowable rotational speed of the wind turbine may be determined and used to control wind turbine operation in accordance with the methods of the invention described above. The rotational speed may be rotor speed, generator speed, or any other suitable rotational speed of the wind turbine. For instance, the rotational speed may be set as the rotor speed, which can then be converted to generator speed through the gear ratio of the wind turbine gearbox. Alternatively, rotational speed may be set as generator speed.

[0077] In the above-described examples, the maximum allowable noise level used in the noise equation / function is set / determined with respect to a defined noise mode describing a relationship between maximum allowable noise level and wind speed. In different examples, the maximum allowable noise level used in the noise equation may be set / determined in any other suitable manner, based on any other suitable parameters. For instance, the maximum allowable noise level may simply be set at each time step - or at each time at which the maximum allowable rotational speed is determined - as desired.

Claims

CLAIMS1. A method of controlling a wind turbine in accordance with a maximum allowable noise level of the wind turbine, the method comprising: defining a noise function describing a relationship between rotational speed of the wind turbine and noise level generated by the wind turbine; defining the maximum allowable noise level, and setting the noise level in the noise function to be equal to the defined maximum allowable noise level; solving the noise function for the rotational speed, with the noise level equal to the defined maximum allowable noise level, to obtain a maximum allowable rotational speed of the wind turbine; and controlling the wind turbine to operate in accordance with the maximum allowable rotational speed.

2. A method according to Claim 1 , wherein the noise function is defined as an equation describing noise level generated by the wind turbine as a function of rotational speed of the wind turbine, and wherein solving the noise function for the rotational speed comprises: inverting the noise function to obtain an inverse noise function that is an equation describing rotational speed of the wind turbine as a function of noise level generated by the wind turbine; and solving the inverse noise function for the rotational speed, with the noise level equal to the defined maximum allowable noise level, to obtain the maximum allowable rotational speed of the wind turbine.

3. A method according to Claim 1 or Claim 2, wherein the relationship between rotational speed and noise level in the noise equation is dependent on an angle of attack of the wind turbine, the method comprising setting the angle of attack in the noise function to be equal to an angle of attack value, wherein the noise function is solved for the rotational speed with the angle of attack equal to the angle of attack value; optionally, wherein the angle of attack is angle of attack at a defined distance along the length of rotor blades of the wind turbine.

4. A method according to Claim 3, wherein the angle of attack value is an estimated angle of attack, the method comprising determining the estimated angle of attack based on wind turbine rotational speed, rotor pitch angle of one or more rotor blades of the wind turbine,wind speed in the vicinity of the wind turbine, a coefficient of thrust of the wind turbine and a radius of a rotor of the wind turbine.

5. A method according to Claim 4 when dependent on Claim 3, wherein determining the estimated angle of attack comprises including a correction term to account for twist of the rotor blades at the defined distance along the length of the rotor blades.

6. A method according to Claim 4 or Claim 5, wherein the rotor pitch angle is a rotor blade pitch reference for controlling the rotor blades, and wherein the rotor blade pitch reference is set to be greater than a threshold pitch angle below which the wind turbine will stall.

7. A wind turbine according to any previous claim, wherein defining the maximum allowable noise level comprises: obtaining a wind speed signal indicative of wind speed in the vicinity of the wind turbine; and based on the obtained wind speed, retrieving the maximum allowable noise level from a lookup table comprising pairs of wind speed and corresponding maximum allowable noise level values.

8. A method according to any previous claim, the method comprising selecting a noise mode from a plurality of defined noise modes, wherein each defined noise mode defines a relationship describing maximum allowable noise level as a function of wind speed, and wherein defining the maximum allowable noise level comprises defining the maximum allowable noise level in accordance with the selected noise mode.

9. A method according to any previous claim, the method comprising: monitoring a measured noise level, using an acoustic sensor, generated by the wind turbine when the wind turbine is being operated in accordance with the maximum allowable rotational speed; determining an error between the measured noise level and the defined maximum allowable noise level; and adjusting the maximum allowable rotational speed to reduce the determined error.

10. A method according to any previous claim, the method comprising, in a training phase: at a plurality of different wind speeds:monitoring measured noise level when the wind turbine is being operated in accordance with the maximum allowable rotational speed, obtained from the noise function, for the respective wind speed; determining a difference between measured noise level and maximum allowable noise level for the respective wind speed; determining a correction value to the maximum allowable rotational speed to reduce the determined difference; fitting a correction curve using the correction values and corresponding wind speeds as interpolation points, the method further comprising controlling the wind turbine to operate in accordance with an updated maximum allowable rotational speed obtained by adjusting the maximum allowable rotational speed, obtained from the noise function, according to a current correction value obtained from the correction curve.11 . A method according to Claim 10, wherein a different correction curve is fitted for each different defined noise mode of the wind turbine or each of one or more defined environmental conditions in which the wind turbine operates.

12. A method according to any previous claim, wherein controlling the wind turbine to operate in accordance with the maximum allowable rotational speed comprises: determining one or both of rotor blade pitch reference and output power reference of the wind turbine based on a wind turbine rotational speed reference, wherein the wind turbine rotational speed reference is set to be no greater than the maximum allowable rotational speed; and adjusting rotor blade pitch and / or output power of the wind turbine in accordance with the respective rotor blade pitch reference and / or output power reference.

13. A method according to Claim 12, wherein if the wind turbine rotational speed reference is less than the maximum allowable rotational speed, then no adjustment of the wind turbine rotational speed reference is performed.

14. A controller for controlling a wind turbine in accordance with a maximum allowable noise level of the wind turbine, the controller being configured to: define a noise function describing a relationship between rotational speed of the wind turbine and noise level generated by the wind turbine;define the maximum allowable noise level, and set the noise level in the noise function to be equal to the defined maximum allowable noise level; solve the noise function for the rotational speed, with the noise level equal to the defined maximum allowable noise level, to obtain a maximum allowable rotational speed of the wind turbine; and control the wind turbine to operate in accordance with the maximum allowable rotational speed.

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

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